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Drainage and valley asymmetry in the Tertiary Hills of Lower Bavaria, Germany
ELSEVIER
Geomorphology 14 (1995) 255-265
Drainage and valley asymmetry in the Tertiary Hills of Lower Bavaria, Germany Rainer Wende School of Geosciences, The University of Wollongong, Wollongong, N.S. W. 2522, Australia
Received 17 May 1994; revised 1 April 1995; accepted 12 May 1995
Abstract Valley asymmetry with steeper west-facing valley sides is a common feature in the Tertiary Hills of Lower Bavaria. Several previous studies have altributed these forms to microclimatic differences under periglacial conditions during the Pleistocene glacials. However, little attention has been given to the pattern of drainage networks in this area as a means of explaining valley asymmetry. Drainage networks show asymmetry both in length and number of tributaries on either side of the main stream. Drainage asymmetry is very likely primarily the reason of a drainage development in competitive situations. Four possible causes have been identifed: ( 1) the position of an initial channel in relation to its adjacent parallel or subparallel drainage lines, (2) different rates of headward erosion of tributaries on either side of an inter-stream divide, (3) drainage development oblique to an initial terrain slope and (4) tilting of a landsurface and the resulting preferential headward erosion of consequent running streams. Thus, asymmetric drainage development causes differences in slope dimensions and an imbalance in run-off and sediment yield on opposite valley sides leading to the development of asymmetric valleys.
1. Introduction Asymmetric valleys are a characteristic feature of the Tertiary Hills of Lower Bavaria. Generally, valley sides facing west are relatively steeper, but valley asymmetry is not limited to north-south trending valleys as relatively steelper valley sides occur facing in every direction (e.g. F’oser and Miiller, 1951). A general lithological or structural control of valley asymmetry in the study area is not evident (Helbig, 1965; Karrasch, 1970). The frequent facies changes in horizontal as well as vertical directions (Figs. 1, 2A, B), the general weakness of the exposed rocks, and the lack of substantial faulting and folding or other well developed geological structures, all rule out variable lithology, structure or warping as general causes for the valley asymmetry i.n the Tertiary Hills. 0169-555X/95/$09.50 8 1095 Elsevier Science B.V. All rights reserved SSDIO169-555X(95)00114-X
Due to a supposed lack of other explanations, most previous studies have attributed the differences in slope angles on opposite valley sides of this region to microclimatic differences under periglacial conditions during the Pleistocene glacials (e.g. Poser and Miiller, 195 1; Helbig, 1965; Karrasch, 1970). It is implied in these theories that a more or less symmetrical valley has evolved into an asymmetric valley due to differences in the slope processes operating on opposing hill slopes and unilateral stream erosion causing a shift of the valley axis towards one side of the valley (Fig. 6A). Any shift of the divides in respect to the streams is not explicitly mentioned. The major reasons for this variation in slope processes and differential fluvial attack has been the subject of persistent disagreement between researchers. Some authors (Biidel, 1944; Helbig, 1965) have suggested
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that solifluction and slope flattening was most active on east-facing slopes during the Pleistocene due to preferential snow accumulation and the resulting soil saturation on valley sides leewards to the prevailing westerly to northwesterly winds. Furthermore, the increased sediment supply from the declining valley sides would push the streams against the western valley sides, causing undercutting and steepening there. Others (e.g. Poser and Mtiller, 195 1) concluded that solifluction must in fact have been more active on west to south-facing slopes causing a “primary valley asymmetry” restricted to the valley headwaters with steeper slopes facing in northerly and easterly directions. Further downvalley unilateral stream erosion and slope steepening on these west- to south-facing slopes would then have been more effective than the slope flattening by solifluction, resulting in a “secondary asymmetry”. The trigger for this lateral shift of the channels was seen in rapid and prolonged thawing of the slope base upon favourable exposure to sunlight. Karrasch ( 1970) supported this proposal of Poser and Miiller ( 195 1) , adding that in valleys with a length of more than approximately 10 km the stream’s capacity to laterally erode must have been high enough to erode both valley sides despite any differences in resistance due to variations in thawing. In larger valleys, relatively steeper slopes could therefore occur on either side of the valley, but still more frequently orientated in southerly to westerly directions. These views have been important in interpretations of fossil valley asymmetries for the entire Pleistocene periglacial area of Middle Europe. However, these theories, although based on evidence from the same region, are contradictory in many major aspects. Even more problematically, Helbig ( 1965) has critically examined the principal hypothesis suggested by Poser and Mtiller ( 195 1) and found it to be not conclusive, but saw the ideas of Biidel ( 1944) confirmed. Karrasch ( 1970) on the other hand, supported Poser and Milller’s theory and rejected Biidel’s and Helbig’s. In contrast to detailed studies of asymmetric valleys in Lower Bavaria, little attention has been given to the drainage networks in this area. It is well known that network evolution and valley development are closely connected and drainage evolution in general is a possible cause of valley asymmetry (Kennedy, 1976). This is especially important in fluvially dissected landscapes where the topography is determined by valleys
and interfluves which are related to each other by the interaction between streams and adjacent hillslopes. During the development of such erosional topography, asymmetric transverse valley profiles have been seen to result from a lateral migration of streams and associated changes of drainage pattern (Schumm, 1956). In early studies, like those of Siilch ( 1918) and Krebs ( 1937), basic causal relations between drainage and valley asymmetry were suggested for the Tertiary Hills and other regions, but as noted, these ideas were disregarded by later authors working on the question of valley asymmetry. Sijlch and Krebs saw the link between drainage and valley asymmetry in unilateral stream erosion caused by variations in sediment supply due to drainage asymmetry. Various possible factors causing asymmetric drainage development were proposed, including deferment of tributary junctions, existing terrain slopes prior to dissection (e.g. large alluvial fans), and tilting of an existing landscape. Underlying geological trends have traditionally been seen as a major reason for the development of certain drainage patterns elsewhere, and a number of studies support this assumption (e.g. Abrahams and Flint, 1983; cf. Mock, 1976). In regions without a geological control on drainage or valley development, other causes such as variations in microclimate (Beaty, 1962), or a forced lateral shift of a trunk stream due to a deflection of the river mouth (White, 1966)) have been suggested. And studies on tributary arrangements and topological properties of channel networks indicate that the spatial requirements of subbasins, available relief and valley winding have a considerable affect on the topological and length properties of drainage networks as well (Abrahams, 1984). In situations where adjacent basins have common divides, the growth rates of these basins will differ due to differing degrees of competition between the basin (Faulkner, 1974), and such network growth in competitive situations can result in drainage asymmetry where tributary development is influenced by the constraint of available area. For example, the development of new streams along major tributaries which enter the parent stream at high angles can be favoured on the obtuse side (upslope side with reference to the parent stream) of the joining tributary (Horton, 1945) or, if the angle is relatively small, on the acute (downslope) side (Flint, 1980), depending upon which side of the tributary area is more limited. Variations in the distance
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between the trunk channel and the divides in general creates an imbalancle of drainage which can result in the asymmetric development of tributaries (Flint, 1980). Moreover, drainage asymmetry as a cause of lateral stream migration and resulting valley asymmetry has been theoretically modelled and simulated by Band ( 1987). In the Isar-Inn Hill Country, a part of the Tertiary Hills of Lower Bavaria, many drainage networks have an asymmetric arrangement of tributaries in relation to the parent stream. ‘This coincides well with valley asymmetry and appears to be a simple explanation for the asymmetry of larger valleys in the area. These drainage asymmetries are probably the result of drainage evolution under various competitive situations, and the purpose of this study is to examine the relation between this drainage and valley asymmetry and to review the possible causes of drainage asymmetry. 2. Regional setting The study area is located in the Isar-Inn Hill Country, a part of the Tertiary Hills of Bavaria in the northern Alpine Foreland (Fig. 1). Here the outcropping unfolded Tertiary sediments of the Upper Freshwater Molasse are unconsolidated and large quantities of material are available for transport. In the southwest of
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this area sands, marls, and clays of the Hangendserie are exposed, while further to the east and north the older coarse elastic sediments of the Northern and Southern Main Gravel dominate (Fig. 1) . The facies change frequently in horizontal as well as vertical direction (Fig. 1 and Fig. 2A, B) . The youngest documented faulting predates the deposition of the uppermost sediments of the Upper Freshwater Molasse (Unger, 1987). During most of the deposition of the Molasse drainage was mainly to the west, but in the east a reversal of the drainage probably began with the termination of the Molasse sedimentation at the end of the Upper Miocene (Unger, 1989). Uplift of the Alpine Foreland in the Pliocene, with a higher rate in the west, resulted in an eastward descending surface on which the drainage of the Danube developed (Fischer, 1989). As a consequence, the evolution of contemporary landforms began with the development of the present day drainage system, probably some 5-7 million years ago. During the Pleistocene glacials the Isar-Inn Hill Country was characterized by periglacial conditions and loess accumulated preferentially on northeast- to east-facing slopes. Underneath the loess, asymmetric valley cross profiles can generally be found which indicate that asymmetric valley development had commenced prior to loess deposition (Karrasch, 1970). Long lasting fluvial erosion in combination with weathering and denudation has resulted in dissection
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0 I
C
5mm 1
25
C’
Fig. 2. (A, B) Generalized geological profiles. (C) Simplified profile through the western part of the study area. Note the different extent of incision of adjacent major drainage lines.
Fig. 3. Drainage network of the study area based on the blue-line network of the topographic maps 1:2CO,OOO. Boxes highlight networks shown in Fig. 4.
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crenulation
259
n e t w o r k “...“’
stream order:
1st e-.__
snd __I
. . -._bi.;/
Fig. 4. Selected asymmetric drainage networks. Blue-line networks, topographic map 1:25,000. Lines indicate the location of the profiles in Fig.
of the landscape into a dense network of branching valleys and chains of rounded hills of low relief. In general, the slopes are gentle with maximum angles often below 10 degrees, but locally steepened slope sections can be as steep as 30 degrees. The crests of the interlluves and major divides are well rounded and the average maximum relief within an area of 25 square kilometres is 80 m.
3. Asymmetry of the drainage networks Fig. 3 shows the general drainage pattern of the study area based on 1:200,000 topographic maps. The main streams flow roughly parallel in north to northeasterly directions and their tributaries generally enter at large angles. These tributaries are often parallel, and in the western part of the s#tudy area northwest-southeast trending tributaries dominate (Fig. 3). The distribution of the tributaries along the northeast to east trending main streams is not symmetric and more numerous and longer tributaries enter the GroRe Vils, for example, from the northwest than from the southeast. The same
is true for the Kleine Vils, Bina and Rott. Further downstream the larger tributaries of the Vils are south of the principal watercourse and the distance between the main channel and the right or left watershed of the basin is therefore unequal. On that side of the stream with the shorter distance to the watershed, the tributary catchments of low order are characterized by shorter basins, a similar basin relief, and resulting higher relief ratios, compared with the tributary basins on the other side of the parent stream. In a more detailed observation (blue-line network of 1:25,000 topographic maps) these tributary catchments of low order themselves show a basin asymmetry similar to that of the larger basins. A good example is the Kirchlemerbach (Fig. 4B) where the larger part of this basin’s area, with the longer tributaries, is north of the trunk stream. Along the Eibach (Fig. 4C) and Gassauer Mtihlbach (Fig. 4A), however, the longer tributaries enter from the west. The most eastern and northern second order tributary of the Kirchlemerbach also has an asymmetric drainage network (Fig. 4B). In the north-south trending tributaries of the north to northeast running main rivers, extension of the drain-
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age networks was apparently favoured in westerly directions. This is readily recognisable on the 1:25,000 topographic maps of the area. Helbig ( 1965) described basin asymmetry as being a common feature for the tributary valleys of the Rott in the Isar-Inn Hill Country, and similarly for other parts of southern Germany and Austria. He stated that in these regions, the northwest-southeast, north-south and northeast-southwest trending valleys receive more frequent and longer dells (elongated denudational slope hollows) from the southwest to northwest than from the southeast to northeast. But the catchments and valleys he studied in the Tsar-Inn Hill Country receive not only more large dells from westerly directions, but also a greater number of longer perennial watercourses.
4. Geometric characteristics of the valleys In catchments of low order, especially first order catchments, there is an obvious geometric relationship between an asymmetric position of the stream and slope asymmetry. Since the valley slopes end at the divides,
the horizontal distance between slope base and slope end is not equal for both valley sides. But for catchments of higher order the slope profiles end at isolated hills along the interfluves and the asymmetric location of the stream is not necessarily directly geometrically related to the horizontal slope length or to any other morphometric slope property. To identify the geometric characteristics of the asymmetric valleys higher than first order in the Isat-Inn hill country, valley cross sections at random locations were examined. A random sample has been used to avoid personal bias. The slopes were determined from the slope base to the slope crest. The maximum slope angle, slope length (horizontal), slope height and slope form were measured from 1:25,000 topographic maps, the accuracy of which for the purpose of this study, was found to be satisfactory, since the general slope characteristics were of more interest than absolute values. Generally, the valleys are characterized by sides with distinctly different slopes. The slopes with the higher maximum slope angle are also significantly shorter than the slopes on the opposite valley sides (Table 1). Slope heights on opposite valley sides are similar, and
Table 1 Slope dimensions of valley cross profiles Valleys with streams of 2nd or 3rd order
Valleys with streams of 4th or higher order
Relatively steeper N= 32
Relatively steeper N=4
Relatively gentler N=32
Height H (m) Mean Standard deviation Standard error f-test significance level
33.4 9.3 1.6 n.s.
34.2 12.9 2.3
33.3 9.3 4.7 ns.
Length (horizontal) L (m) Mean Standard deviation Standard error t-test significance level
515 223.4 39.5 CO.01
851 398.8 70.5
538 168.9 84.5 0.05
Mean slope angle HIL Mean Standard deviation Standard error t-test significance level
0.07 0.027 0.005 CO.01
0.044
0.064
0.016 0.003
0.013 0.006 0.05
Relatively gentler N=4
36.5 6.2 3.1
1663 595.3 297.7
0.025 0.011 0.006
Profiles are grouped into relatively steeper and gentler slopes based on the observed maximum slope angles. The mean, standard deviation and standard error were determined for each group. The significance of the observed differences between the means is given as a t-test significance level. N = No. of observations.
R. Wende IGeomorphology 14 (1995) 255-265
the shorter slope with the higher maximum slope angle is, therefore, the slope with the higher average slope angle as well. The geometric properties of the northeast to east trending valle:ys of higher order are found to be the same as for their tributary valleys of lower order (Table 1). The form of the slopes are most often straight to slightly convex, although many complex slope forms do occur. Concave segments at the foot of the slopes are, if present, only short. Convex breaks in slope are prominent in the lower parts of the slopes and are common on slopes located opposite to where larger tributaries enter, or near the mouth of tributary catchments on the downstream side of the valley with reference to the parent stream. In the field these steepened lower slope segments show signs of recent earth slides and the channels are frequently located at the immediate base of these slopes. Valley side slopes are commonly segmented laterally into alternating spurs and hollows and areas with straight contours are rare, seeming to occur more often on the steeper valley side where the divide is closer to the main channel. Presumably, slope length is below critical distance required for erosion and channel development (cf. Horton, 1945; Dunne, 1980). Along the sides of valleys of higher order, slopes that drain directly into these valleys are often restricted to the mouths of the catchments or the inter-basin areas which are generally the lower parts of interfluves with laterally convex extending slopes.
5. Drainage asymmetry and valley asymmetry Asymmetry in greater than first order valleys in the Isar-Inn Hill Country is easily identifiable in the 1:25,000 topographic maps. A relation between drainage asymmetry and v,alley asymmetry is also obvious. The shorter and steeper slopes are on the side of the valley with the fewer and shorter feeder streams and shorter distances to the watershed. The relatively steeper valley side of the Kleine (Kl.) Vils, GroBe (Gr.) Vils and Rott is south of these rivers (Fig. 3). Downstream of the confluence of Kl. Vils and Gr. Vils the steeper valley sidle of the Vils switches with the decreasing distance between northern watershed and channel to the northern side. The steeper valley sides along the Bina in a downstream direction face west then northwest, north and finally northeast, always
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being on the southern side of the stream with a shorter distance to the watershed. Just before its confluence with the Rott, the steeper slopes switch to the northern side of the valley, facing southwest. Basins of lower order show the same coincidence between drainage network or basin asymmetry and valley asymmetry as do basins of high order (Fig. 4). Along the valley of the Eibach, the steeper valley side is facing generally westward (Fig. 5C); along the Kirchlemerbach it is facing to the north (Fig. 5B). On the upper Gassauer Mtihlbach the steeper valley side of the second order segment faces to the west (Fig. 5A), but, after the valley’s distinctive change of direction, it faces to the south. In the westernmost second order tributary of the Kirchlemerbach, the steeper valley side faces to the southeast (Fig. 4B). Beside valley-wide asymmetries characterized by different maximum and average slope angles and different slope lengths on opposite valley sides, there are also localized asymmetries. These occur when the shorter valley side with the smaller mean slope angle is locally steepened at the lower parts of the slope, resulting in a higher maximum slope angle than on the opposite valley side.
6. Discussion Differences in slope length in valleys with a similar slope height on opposite sides are a characteristic of the study area. Within the asymmetric valleys, the shorter slopes are also generally steeper. Although these differences in slope dimensions can cause “autoasymmetry”, they are not an explanation for valley asymmetries since they are themselves the result of other factors (Kennedy, 1976). Because of the remarkable coincidence between drainage asymmetry and valley asymmetry throughout the region it seems very likely that the evolution of the asymmetric valleys is related to the evolution of the drainage networks. The most obvious and best documented controls on specific drainage patterns are lithology and geological structure. In the study area, tributary basins and their parent basins are often further extended into directions transverse to each other. This can hardly be explained by a variation in lithology or tilted strata in the presence of frequently changing facies and the absence of dipping beds extending over areas suffi-
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262
A
s 5 0 0 -r % 4 6 0 -P d Y 4 6 0 --
500
,460 &
T
--
s g 460 -h Y
440..
420 1 5000
4000
3000
2000
1000
0
-1000
-2000
Dirhnee [ml Fig. 5. Opposite interfluve (solid boxes) and adjacent tributary valley profiles (unfilled boxes) for a 2nd, 3rd and 4th order valley (Fig. 4). The broken line (diamonds) is the inverted mirror image of the valley profile. Note the remarkable line fit along the lower parts of the intertluves.
ciently large enough to enhance a systematic uniclinal shifting of strike streams (Figs. 1, 2A, B). However, as for the asymmetric valleys (e.g. Karrasch, 1970)) a general structural or lithological control of the drainage pattern in the study area seems to be very unlikely. A microclimatic influence on drainage evolution has been suggested in studies from other places (e.g. Beaty, 1962). In the Tertiary Hills, under periglacial conditions the microclimatic variations on slopes of different orientation, a somewhat controversial issue (e.g. Poser and Mtiller, 1951; Helbig, 1965), might have been
sufficiently strong enough to influence the development of drainage in smaller basins, but the disagreement about the nature and rate of processes on slopes of different exposure within the area (c.f. Poser and Miiller, 195 1; Helbig, 1965) seems to indicate that local variations in intensity were high and differences due to variation in exposure not very pronounced. Furthermore, drainage and associated valley asymmetries like those of the Kirchlemer Bach (Fig. 4B), or the larger northeast to east running streams (Fig. 3), are clearly not a result of preferential lateral erosion due to
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favourable exposure to sunlight since their steeper and shorter valley sides face in northerly directions. Also the changing location of the larger part of the drainage area of the Gr. Vils and Rott Rivers in a downstream direction from the northern side of the channel to southern side (Fig. 3) contradicts the idea that prevailing northwesterly to westerly winds had considerable influence on their drainage evolution. If drainage asymmetry is assumed to be the result of a lateral shift of the main channel to one side of the basin then streams like the Rott upstreamof the junction with the Bina (Fig. 3) would have shifted some 5 to 6 km to one side without leaving any evidence for this evolution, except for the asymmetry of the drainage itself. Any variations, in microclimate powerful enough to trigger a process like this should have resulted in a similar shift of the Kollbach, located just a few kilometers northeast of the Rott. The same line of reasoning can be applied to question an assumed “climatically” triggered differential headward extension of the tributaries instead of a lateral shift of the main channel. As neither lithological variability and geological structure, nor variations in microclimate appear to be plausible explanations for drainage asymmetries in the study area, the cause of the asymmetry especially of the larger valleys remains questionable. An alternative explanation is thus required. In the Isa-Inn Hill Country, the differences in slope dimensions are closely related to the arrangement of the drainage networks. In valleys where the slopes extend from the valley floor to the watershed, the slope height and slope length is directly restricted by the distance between the valley floor and adjacent watershed. In valleys of higher order the slopes end frequently on isolated hills along interior watersheds. The profiles are located in the inter-basin areas between adjacent tributary basins and are the lower parts of interfluves. Slope height and slope length are therefore determined by the profiles of the latter. The interfluves themselves are functionally related to the adjacent valleys, and in fluvially dissected regions their longitudinal profiles are an approximate inverted mirror image of the valley profiles (Ahnert, 1984). Since opposite tributary basins in the study area have approximately the same relief but a different length on either side of the parent stream, the logarithmic profiles of the valleys and the related profiles of the interfluves are steeper on that side of the parent stream where the distance to the
263
Fig. 6. Asymmetric valley development assuming: (A) a lateral shift of the valley axis, (B) asymmetric extension of tributary valleys on opposite sides of the main valley. Resulting tributary valley profiles (dashed lines) and interlluve profiles (solid lines) have different gradients on opposite sides of the main valley. Interfluve profiles coincide with the slopes of the main valley.
watershed is shorter (Fig. 5). Furthermore, the slopes of the larger valleys are laterally dissected into alternating spurs and hollows, or interfluves and valleys, and as a consequence the valley sides with the steeper interfluve profiles also have the steeper slope profiles. In other words, shorter and therefore steeper tributary valleys have steeper intefluves associated with them. Therefore, a differential extension of the tributary valleys on either side of the main valley generates an asymmetric main valley (Fig. 6B). Drainage and main valley asymmetry are not only linked through a functional relation between interfluves and adjacent valleys, but also through the influence of the tributary stream on the lateral position of the main stream. In catchments or valleys with closely spaced tributaries which are relatively large in respect with the parent stream, drainage asymmetry is a simple explanation for the maintenance of valley asymmetries. Melton ( 1960) found that the position and activity of the channel in respect to the valley-side slopes is one direct cause of valley asymmetry. In many valleys of the IsarInn Hill Country the principal stream receives longer and more numerous tributaries from one valley side. Alluvial fans at the mouths of larger tributaries, the position of the larger channels at the base of the opposite slope, and the frequent local steepening of this side of the valley indicate that the entering tributaries frequently divert the main stream to the opposite valley
264
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side. Indeed, recent earth slides on the steepened lower slope segments are evidence of the activity of basal erosion under present conditions. The characteristic imbalance of feeder streams in asymmetric drainage networks probably causes a diversion of the main channel to the opposite valley side causing a series of locally steepened slopes there. A similar relation between drainage and valley asymmetry was suggested by Siilch (1918). Drainage asymmetry causing differences in slope dimensions and an imbalance in run-off and sediment supply on opposite valley sides appears to be a logical explanation for the lack of symmetry of valleys in the Isar-Inn Hill Country. However, the question as to what causes drainage pattern asymmetry remains. A major factor influencing drainage patterns is the spatial arrangement of the drainage networks. Where this arrangement involves evolution of drainage in certain competitive situations, asymmetric drainage can be produced. Four hypotheses are suggested as possible causes for the drainage asymmetries in the study area: (1) The position of a channel relative to adjacent parallel or subparallel drainage lines restricts the extension of the catchment, particularly in the direction of its closest neighbour, and may therefore result in drainage asymmetry (Fig. 7A). A special case of this situation is the differential tributary development near the junction of subparallel basin as described by Flint (1980). ( 2) Different rates of headward erosion of tributaries on either side of an inter-stream divide, caused by different rates of downcutting of their parallel or subparallel parent streams, may lead to a gradual shift of the dividing watershed towards the minor channel (Fig. 7B). The position of the watershed of the upper Rott is very likely the result of drainage area losses to the adjacent Isen, which has a considerable erosional advantage. Other drainage networks in the study area indicate the same process (Figs. 3 and 2C). Eventually different rates of downcutting lead to capturing which can cause not only drainage but also valley asymmetry (Kennedy, 1976). The sudden change of direction and its associated valley asymmetry along the Gassauer Mtihlbach appears to be the product of stream capture since there is no indication for a lithological or structural control (Fig. 4A). The apparently preferred extension of the smaller north-south trending basins in westerly directions
Fig. 7. Competitive network development and resulting valley asymmetry. (A) Different spacing of parallel/subparallel major drainage lines. (B) Different rates of downcutting. (C) Development of tributaries oblique to an initial terrain slope (modified after Horton, 1945).
could be the result of two further special forms of drainage development in competitive situations: (3) Horton ( 1945) argued that on a surface with low relief and an initial slope, the tributary of a consequent parent stream that runs oblique to the original terrain slope will extend its catchment further to the upslope side than to the downslope side because of enhanced catchment areas and gradients in this direction. The enlargement of the basin and the development of a new generation of tributaries on the valley sides of the parent valley could therefore result in an asymmetric drainage network, with more numerous and longer tributaries upslope of the parent stream (Fig. 7C). (4) Tilting of an existing landsurface could also cause preferential headward erosion of consequent running tributaries and finally drainage asymmetry (Siilch, 1918)) particularly in the Isar-Inn Hill Country (Krebs 1937). However, there is no direct evidence of tilting, although the Pliocene uplift of the Alpine Foreland might have resulted in an initial terrain slope dipping northeast to east for the area of the Isar-Inn Hill Country and could have possibly provided the conditions necessary for the development of asymmetric drainage networks.
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7. Conclusion The differences of’ slope dimensions of the asymmetric valleys in the Isar-Inn Hill Country, and the striking coincidence with the drainage asymmetry, supports the hypothesis that drainage development is a major cause of valley asymmetries. The link between drainage and valley asymmetry in densely fluvially dissected landscapes is the functional relationship between valleys and a.djacent interlluves. The slopes of larger valleys are dissected into alternating valleys and interlluves and are, therefore, related to the characteristics of the drainage network. Furthermore, asymmetric drainage causes asymmetric sediment yield from opposite valley sides and may result in preferential lateral erosion on one valley side. There are several possible causes for drainage asymmetry: the position of an initial channel to its adjacent parallel or subparallel drainage lines; the different rates of headward erosion of tributaries on either side of an inter-stream divide; drainage development oblique to an initial terrain slope; and tilting of a land surface and resulting preferential headward erosion of consequent running streams. The contemporary drainage of densely fluvially dissected landscapes with a long denudational history, is the likely result of an evolution in competitive situations, but due to the complexity of this evolution, it is certainly difficult and often impossible to reconstruct individual evolutions. Nevertheless, any study on valley asymmetry in these landscapes should investigate the relation between drainage evolution and valley development before hypothesised palaeomicroclimatic differences are considered as a means of explaining valley asymmetries.
Acknowledgements The author is especially grateful to F. Ahnert, W. Romer and G. Nans~on for valuable comments. The comments of B.A. Kennedy and an anonymous reviewer helped to improve the manuscript. D. Martin, University of Wollongong, produced the final diagrams. This study was prepared at the Department of Geography, RWTH Aachen, Germany. References Abrahams, A.D., 1984. Ch,mnel networks: a geomorphological perspective. Water Resour. Res., 20: 161-168.
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Abrahams, A.D. and Flint, J.J., 1983. Geological controls on the topological properties of some trellis channel networks. Geol. Sot. Am. Bull., 94: 80-91. Ahnert, F., 1984. Local relief and the height limits of mountain ranges. Am. _I. Sci., 284: 103.5-1055. Band, L.E., 1987. Lateral migration of stream channels. Catena Suppl., 10: 99-l 10. Beaty, C.B., 1962. Asymmetry of stream patterns and topography in the Bitterroot Range, Montana. J. Geol., 70: 347-354. Bildel, J., 1944. Die morphologischen Wirkungen des Eiszeitenklimas im gletscherfreien Gebiet. Geol. Rundsch., 34: 482-5 19. Dunne, T., 1980. Formation and controls of channel networks. Prog. Phys. Geogr., 4: 21 l-239. Faulkner, H., 1974. An allometric growth model for competitive gullies. Z. Geomorphol. Suppl., 21: 7687. Fischer, K., 1989. The landforms of the German Alps and the Alpine Foreland. Catena Suppl., 15: 69-83. Flint, J.J., 1980. Tributary arrangements in fluvial systems. Am. J. Sci., 280: 2645. Helbig, K., 1965. AsymmettischeEiszeittalerin Sttddeutschland und Gsterreich. W&burger Geographische Arbeiten, 14, Wilrzburg. Horton, R.E., 1945. Erosional development of streams and their drainage basins: hydrophysical approach to quantitative morphology. Geol. Sot. Am. Bull., 56: 275-370. Karrasch, H., 1970. Das Phanomen der klimabedingten Reliefasymmetrie in Mitteleuropa. Giittinger Geogr. Abh., 56, Gottingen. Kennedy, B.A., 1976. Valley-side slopes and climate. In: E. Derbyshire (Editor), Geomorphology and Climate. London, pp. 17 l201. Krebs, N., 1937. Talnetzstudien. Sitzungsberichte der PreuBischen Akademie der Wissenschaften, Sitzung der physikalisch-mathematischen Klasse, VI. Melton, M.A., 1960. Intravalley variation in slope angles related to microclimate and erosional environment. Geol. Sot. Am. Bull., 71: 133-144. Mock, S.J., 1976: Topological properties of some trellis pattern channel networks. U.S. Army Cold Regions Research and Engineering Laboratory Report 76-46, Hanover, N.H.. Poser, H. and Mtlller, T., 1951. Studien an den asymmetrischen Talem des Niederbayerischen Htlgellandes. Nachrichten der Akademie der Wissenschaften in Gottingen, Mathematisch-Physikalische Klasse, Biologisch-Physiologisch-Chemische Abteilung, Nr. 1, Gottingen. Schumm, S.A., 1956. Evolution of drainage systems and slopes in badlands at Perth Amboy, New Jersey. Geol. Sot. Am. Bull., 67: 597-646. Solch, J., 1918. Ungleichseitige FluDgebiete und Talquerschnitte. Petermanns Geographische Mitteilungen, 64, Gotha. Unger, H.J., 1987. Bemerkungen zum tektonischen Werdegang Stldostbayems. Geol. Jahrbuch, A 105: 3-23. Unger, H.J., 1989. Die Lithozonen der Oberen SttBwassermolasse Stidostbayems und ibre vermutlichen zeitlichen Aquivalente gegen Westen und Osten. Geologica Bavarica, 94: 194-237. White, W.A., 1966. Drainage asymmetry and the Carolina Capes. Geol. Sot. Am. Bull., 77: 223-240.
Geomorphology 14 (1995) 255-265
Drainage and valley asymmetry in the Tertiary Hills of Lower Bavaria, Germany Rainer Wende School of Geosciences, The University of Wollongong, Wollongong, N.S. W. 2522, Australia
Received 17 May 1994; revised 1 April 1995; accepted 12 May 1995
Abstract Valley asymmetry with steeper west-facing valley sides is a common feature in the Tertiary Hills of Lower Bavaria. Several previous studies have altributed these forms to microclimatic differences under periglacial conditions during the Pleistocene glacials. However, little attention has been given to the pattern of drainage networks in this area as a means of explaining valley asymmetry. Drainage networks show asymmetry both in length and number of tributaries on either side of the main stream. Drainage asymmetry is very likely primarily the reason of a drainage development in competitive situations. Four possible causes have been identifed: ( 1) the position of an initial channel in relation to its adjacent parallel or subparallel drainage lines, (2) different rates of headward erosion of tributaries on either side of an inter-stream divide, (3) drainage development oblique to an initial terrain slope and (4) tilting of a landsurface and the resulting preferential headward erosion of consequent running streams. Thus, asymmetric drainage development causes differences in slope dimensions and an imbalance in run-off and sediment yield on opposite valley sides leading to the development of asymmetric valleys.
1. Introduction Asymmetric valleys are a characteristic feature of the Tertiary Hills of Lower Bavaria. Generally, valley sides facing west are relatively steeper, but valley asymmetry is not limited to north-south trending valleys as relatively steelper valley sides occur facing in every direction (e.g. F’oser and Miiller, 1951). A general lithological or structural control of valley asymmetry in the study area is not evident (Helbig, 1965; Karrasch, 1970). The frequent facies changes in horizontal as well as vertical directions (Figs. 1, 2A, B), the general weakness of the exposed rocks, and the lack of substantial faulting and folding or other well developed geological structures, all rule out variable lithology, structure or warping as general causes for the valley asymmetry i.n the Tertiary Hills. 0169-555X/95/$09.50 8 1095 Elsevier Science B.V. All rights reserved SSDIO169-555X(95)00114-X
Due to a supposed lack of other explanations, most previous studies have attributed the differences in slope angles on opposite valley sides of this region to microclimatic differences under periglacial conditions during the Pleistocene glacials (e.g. Poser and Miiller, 195 1; Helbig, 1965; Karrasch, 1970). It is implied in these theories that a more or less symmetrical valley has evolved into an asymmetric valley due to differences in the slope processes operating on opposing hill slopes and unilateral stream erosion causing a shift of the valley axis towards one side of the valley (Fig. 6A). Any shift of the divides in respect to the streams is not explicitly mentioned. The major reasons for this variation in slope processes and differential fluvial attack has been the subject of persistent disagreement between researchers. Some authors (Biidel, 1944; Helbig, 1965) have suggested
256
R. Wende / Geomorphology 14 (1995) 25.5-265
that solifluction and slope flattening was most active on east-facing slopes during the Pleistocene due to preferential snow accumulation and the resulting soil saturation on valley sides leewards to the prevailing westerly to northwesterly winds. Furthermore, the increased sediment supply from the declining valley sides would push the streams against the western valley sides, causing undercutting and steepening there. Others (e.g. Poser and Mtiller, 195 1) concluded that solifluction must in fact have been more active on west to south-facing slopes causing a “primary valley asymmetry” restricted to the valley headwaters with steeper slopes facing in northerly and easterly directions. Further downvalley unilateral stream erosion and slope steepening on these west- to south-facing slopes would then have been more effective than the slope flattening by solifluction, resulting in a “secondary asymmetry”. The trigger for this lateral shift of the channels was seen in rapid and prolonged thawing of the slope base upon favourable exposure to sunlight. Karrasch ( 1970) supported this proposal of Poser and Miiller ( 195 1) , adding that in valleys with a length of more than approximately 10 km the stream’s capacity to laterally erode must have been high enough to erode both valley sides despite any differences in resistance due to variations in thawing. In larger valleys, relatively steeper slopes could therefore occur on either side of the valley, but still more frequently orientated in southerly to westerly directions. These views have been important in interpretations of fossil valley asymmetries for the entire Pleistocene periglacial area of Middle Europe. However, these theories, although based on evidence from the same region, are contradictory in many major aspects. Even more problematically, Helbig ( 1965) has critically examined the principal hypothesis suggested by Poser and Mtiller ( 195 1) and found it to be not conclusive, but saw the ideas of Biidel ( 1944) confirmed. Karrasch ( 1970) on the other hand, supported Poser and Milller’s theory and rejected Biidel’s and Helbig’s. In contrast to detailed studies of asymmetric valleys in Lower Bavaria, little attention has been given to the drainage networks in this area. It is well known that network evolution and valley development are closely connected and drainage evolution in general is a possible cause of valley asymmetry (Kennedy, 1976). This is especially important in fluvially dissected landscapes where the topography is determined by valleys
and interfluves which are related to each other by the interaction between streams and adjacent hillslopes. During the development of such erosional topography, asymmetric transverse valley profiles have been seen to result from a lateral migration of streams and associated changes of drainage pattern (Schumm, 1956). In early studies, like those of Siilch ( 1918) and Krebs ( 1937), basic causal relations between drainage and valley asymmetry were suggested for the Tertiary Hills and other regions, but as noted, these ideas were disregarded by later authors working on the question of valley asymmetry. Sijlch and Krebs saw the link between drainage and valley asymmetry in unilateral stream erosion caused by variations in sediment supply due to drainage asymmetry. Various possible factors causing asymmetric drainage development were proposed, including deferment of tributary junctions, existing terrain slopes prior to dissection (e.g. large alluvial fans), and tilting of an existing landscape. Underlying geological trends have traditionally been seen as a major reason for the development of certain drainage patterns elsewhere, and a number of studies support this assumption (e.g. Abrahams and Flint, 1983; cf. Mock, 1976). In regions without a geological control on drainage or valley development, other causes such as variations in microclimate (Beaty, 1962), or a forced lateral shift of a trunk stream due to a deflection of the river mouth (White, 1966)) have been suggested. And studies on tributary arrangements and topological properties of channel networks indicate that the spatial requirements of subbasins, available relief and valley winding have a considerable affect on the topological and length properties of drainage networks as well (Abrahams, 1984). In situations where adjacent basins have common divides, the growth rates of these basins will differ due to differing degrees of competition between the basin (Faulkner, 1974), and such network growth in competitive situations can result in drainage asymmetry where tributary development is influenced by the constraint of available area. For example, the development of new streams along major tributaries which enter the parent stream at high angles can be favoured on the obtuse side (upslope side with reference to the parent stream) of the joining tributary (Horton, 1945) or, if the angle is relatively small, on the acute (downslope) side (Flint, 1980), depending upon which side of the tributary area is more limited. Variations in the distance
R. Wende / Geomorphology 14 (1995) 255-265
between the trunk channel and the divides in general creates an imbalancle of drainage which can result in the asymmetric development of tributaries (Flint, 1980). Moreover, drainage asymmetry as a cause of lateral stream migration and resulting valley asymmetry has been theoretically modelled and simulated by Band ( 1987). In the Isar-Inn Hill Country, a part of the Tertiary Hills of Lower Bavaria, many drainage networks have an asymmetric arrangement of tributaries in relation to the parent stream. ‘This coincides well with valley asymmetry and appears to be a simple explanation for the asymmetry of larger valleys in the area. These drainage asymmetries are probably the result of drainage evolution under various competitive situations, and the purpose of this study is to examine the relation between this drainage and valley asymmetry and to review the possible causes of drainage asymmetry. 2. Regional setting The study area is located in the Isar-Inn Hill Country, a part of the Tertiary Hills of Bavaria in the northern Alpine Foreland (Fig. 1). Here the outcropping unfolded Tertiary sediments of the Upper Freshwater Molasse are unconsolidated and large quantities of material are available for transport. In the southwest of
257
this area sands, marls, and clays of the Hangendserie are exposed, while further to the east and north the older coarse elastic sediments of the Northern and Southern Main Gravel dominate (Fig. 1) . The facies change frequently in horizontal as well as vertical direction (Fig. 1 and Fig. 2A, B) . The youngest documented faulting predates the deposition of the uppermost sediments of the Upper Freshwater Molasse (Unger, 1987). During most of the deposition of the Molasse drainage was mainly to the west, but in the east a reversal of the drainage probably began with the termination of the Molasse sedimentation at the end of the Upper Miocene (Unger, 1989). Uplift of the Alpine Foreland in the Pliocene, with a higher rate in the west, resulted in an eastward descending surface on which the drainage of the Danube developed (Fischer, 1989). As a consequence, the evolution of contemporary landforms began with the development of the present day drainage system, probably some 5-7 million years ago. During the Pleistocene glacials the Isar-Inn Hill Country was characterized by periglacial conditions and loess accumulated preferentially on northeast- to east-facing slopes. Underneath the loess, asymmetric valley cross profiles can generally be found which indicate that asymmetric valley development had commenced prior to loess deposition (Karrasch, 1970). Long lasting fluvial erosion in combination with weathering and denudation has resulted in dissection
R. Wende / Geomorphology 14 (1995) 255-265
258
0 I
C
5mm 1
25
C’
Fig. 2. (A, B) Generalized geological profiles. (C) Simplified profile through the western part of the study area. Note the different extent of incision of adjacent major drainage lines.
Fig. 3. Drainage network of the study area based on the blue-line network of the topographic maps 1:2CO,OOO. Boxes highlight networks shown in Fig. 4.
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crenulation
259
n e t w o r k “...“’
stream order:
1st e-.__
snd __I
. . -._bi.;/
Fig. 4. Selected asymmetric drainage networks. Blue-line networks, topographic map 1:25,000. Lines indicate the location of the profiles in Fig.
of the landscape into a dense network of branching valleys and chains of rounded hills of low relief. In general, the slopes are gentle with maximum angles often below 10 degrees, but locally steepened slope sections can be as steep as 30 degrees. The crests of the interlluves and major divides are well rounded and the average maximum relief within an area of 25 square kilometres is 80 m.
3. Asymmetry of the drainage networks Fig. 3 shows the general drainage pattern of the study area based on 1:200,000 topographic maps. The main streams flow roughly parallel in north to northeasterly directions and their tributaries generally enter at large angles. These tributaries are often parallel, and in the western part of the s#tudy area northwest-southeast trending tributaries dominate (Fig. 3). The distribution of the tributaries along the northeast to east trending main streams is not symmetric and more numerous and longer tributaries enter the GroRe Vils, for example, from the northwest than from the southeast. The same
is true for the Kleine Vils, Bina and Rott. Further downstream the larger tributaries of the Vils are south of the principal watercourse and the distance between the main channel and the right or left watershed of the basin is therefore unequal. On that side of the stream with the shorter distance to the watershed, the tributary catchments of low order are characterized by shorter basins, a similar basin relief, and resulting higher relief ratios, compared with the tributary basins on the other side of the parent stream. In a more detailed observation (blue-line network of 1:25,000 topographic maps) these tributary catchments of low order themselves show a basin asymmetry similar to that of the larger basins. A good example is the Kirchlemerbach (Fig. 4B) where the larger part of this basin’s area, with the longer tributaries, is north of the trunk stream. Along the Eibach (Fig. 4C) and Gassauer Mtihlbach (Fig. 4A), however, the longer tributaries enter from the west. The most eastern and northern second order tributary of the Kirchlemerbach also has an asymmetric drainage network (Fig. 4B). In the north-south trending tributaries of the north to northeast running main rivers, extension of the drain-
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260
age networks was apparently favoured in westerly directions. This is readily recognisable on the 1:25,000 topographic maps of the area. Helbig ( 1965) described basin asymmetry as being a common feature for the tributary valleys of the Rott in the Isar-Inn Hill Country, and similarly for other parts of southern Germany and Austria. He stated that in these regions, the northwest-southeast, north-south and northeast-southwest trending valleys receive more frequent and longer dells (elongated denudational slope hollows) from the southwest to northwest than from the southeast to northeast. But the catchments and valleys he studied in the Tsar-Inn Hill Country receive not only more large dells from westerly directions, but also a greater number of longer perennial watercourses.
4. Geometric characteristics of the valleys In catchments of low order, especially first order catchments, there is an obvious geometric relationship between an asymmetric position of the stream and slope asymmetry. Since the valley slopes end at the divides,
the horizontal distance between slope base and slope end is not equal for both valley sides. But for catchments of higher order the slope profiles end at isolated hills along the interfluves and the asymmetric location of the stream is not necessarily directly geometrically related to the horizontal slope length or to any other morphometric slope property. To identify the geometric characteristics of the asymmetric valleys higher than first order in the Isat-Inn hill country, valley cross sections at random locations were examined. A random sample has been used to avoid personal bias. The slopes were determined from the slope base to the slope crest. The maximum slope angle, slope length (horizontal), slope height and slope form were measured from 1:25,000 topographic maps, the accuracy of which for the purpose of this study, was found to be satisfactory, since the general slope characteristics were of more interest than absolute values. Generally, the valleys are characterized by sides with distinctly different slopes. The slopes with the higher maximum slope angle are also significantly shorter than the slopes on the opposite valley sides (Table 1). Slope heights on opposite valley sides are similar, and
Table 1 Slope dimensions of valley cross profiles Valleys with streams of 2nd or 3rd order
Valleys with streams of 4th or higher order
Relatively steeper N= 32
Relatively steeper N=4
Relatively gentler N=32
Height H (m) Mean Standard deviation Standard error f-test significance level
33.4 9.3 1.6 n.s.
34.2 12.9 2.3
33.3 9.3 4.7 ns.
Length (horizontal) L (m) Mean Standard deviation Standard error t-test significance level
515 223.4 39.5 CO.01
851 398.8 70.5
538 168.9 84.5 0.05
Mean slope angle HIL Mean Standard deviation Standard error t-test significance level
0.07 0.027 0.005 CO.01
0.044
0.064
0.016 0.003
0.013 0.006 0.05
Relatively gentler N=4
36.5 6.2 3.1
1663 595.3 297.7
0.025 0.011 0.006
Profiles are grouped into relatively steeper and gentler slopes based on the observed maximum slope angles. The mean, standard deviation and standard error were determined for each group. The significance of the observed differences between the means is given as a t-test significance level. N = No. of observations.
R. Wende IGeomorphology 14 (1995) 255-265
the shorter slope with the higher maximum slope angle is, therefore, the slope with the higher average slope angle as well. The geometric properties of the northeast to east trending valle:ys of higher order are found to be the same as for their tributary valleys of lower order (Table 1). The form of the slopes are most often straight to slightly convex, although many complex slope forms do occur. Concave segments at the foot of the slopes are, if present, only short. Convex breaks in slope are prominent in the lower parts of the slopes and are common on slopes located opposite to where larger tributaries enter, or near the mouth of tributary catchments on the downstream side of the valley with reference to the parent stream. In the field these steepened lower slope segments show signs of recent earth slides and the channels are frequently located at the immediate base of these slopes. Valley side slopes are commonly segmented laterally into alternating spurs and hollows and areas with straight contours are rare, seeming to occur more often on the steeper valley side where the divide is closer to the main channel. Presumably, slope length is below critical distance required for erosion and channel development (cf. Horton, 1945; Dunne, 1980). Along the sides of valleys of higher order, slopes that drain directly into these valleys are often restricted to the mouths of the catchments or the inter-basin areas which are generally the lower parts of interfluves with laterally convex extending slopes.
5. Drainage asymmetry and valley asymmetry Asymmetry in greater than first order valleys in the Isar-Inn Hill Country is easily identifiable in the 1:25,000 topographic maps. A relation between drainage asymmetry and v,alley asymmetry is also obvious. The shorter and steeper slopes are on the side of the valley with the fewer and shorter feeder streams and shorter distances to the watershed. The relatively steeper valley side of the Kleine (Kl.) Vils, GroBe (Gr.) Vils and Rott is south of these rivers (Fig. 3). Downstream of the confluence of Kl. Vils and Gr. Vils the steeper valley sidle of the Vils switches with the decreasing distance between northern watershed and channel to the northern side. The steeper valley sides along the Bina in a downstream direction face west then northwest, north and finally northeast, always
261
being on the southern side of the stream with a shorter distance to the watershed. Just before its confluence with the Rott, the steeper slopes switch to the northern side of the valley, facing southwest. Basins of lower order show the same coincidence between drainage network or basin asymmetry and valley asymmetry as do basins of high order (Fig. 4). Along the valley of the Eibach, the steeper valley side is facing generally westward (Fig. 5C); along the Kirchlemerbach it is facing to the north (Fig. 5B). On the upper Gassauer Mtihlbach the steeper valley side of the second order segment faces to the west (Fig. 5A), but, after the valley’s distinctive change of direction, it faces to the south. In the westernmost second order tributary of the Kirchlemerbach, the steeper valley side faces to the southeast (Fig. 4B). Beside valley-wide asymmetries characterized by different maximum and average slope angles and different slope lengths on opposite valley sides, there are also localized asymmetries. These occur when the shorter valley side with the smaller mean slope angle is locally steepened at the lower parts of the slope, resulting in a higher maximum slope angle than on the opposite valley side.
6. Discussion Differences in slope length in valleys with a similar slope height on opposite sides are a characteristic of the study area. Within the asymmetric valleys, the shorter slopes are also generally steeper. Although these differences in slope dimensions can cause “autoasymmetry”, they are not an explanation for valley asymmetries since they are themselves the result of other factors (Kennedy, 1976). Because of the remarkable coincidence between drainage asymmetry and valley asymmetry throughout the region it seems very likely that the evolution of the asymmetric valleys is related to the evolution of the drainage networks. The most obvious and best documented controls on specific drainage patterns are lithology and geological structure. In the study area, tributary basins and their parent basins are often further extended into directions transverse to each other. This can hardly be explained by a variation in lithology or tilted strata in the presence of frequently changing facies and the absence of dipping beds extending over areas suffi-
R. Wende/Geomorphology 14 (1995) 255-265
262
A
s 5 0 0 -r % 4 6 0 -P d Y 4 6 0 --
500
,460 &
T
--
s g 460 -h Y
440..
420 1 5000
4000
3000
2000
1000
0
-1000
-2000
Dirhnee [ml Fig. 5. Opposite interfluve (solid boxes) and adjacent tributary valley profiles (unfilled boxes) for a 2nd, 3rd and 4th order valley (Fig. 4). The broken line (diamonds) is the inverted mirror image of the valley profile. Note the remarkable line fit along the lower parts of the intertluves.
ciently large enough to enhance a systematic uniclinal shifting of strike streams (Figs. 1, 2A, B). However, as for the asymmetric valleys (e.g. Karrasch, 1970)) a general structural or lithological control of the drainage pattern in the study area seems to be very unlikely. A microclimatic influence on drainage evolution has been suggested in studies from other places (e.g. Beaty, 1962). In the Tertiary Hills, under periglacial conditions the microclimatic variations on slopes of different orientation, a somewhat controversial issue (e.g. Poser and Mtiller, 1951; Helbig, 1965), might have been
sufficiently strong enough to influence the development of drainage in smaller basins, but the disagreement about the nature and rate of processes on slopes of different exposure within the area (c.f. Poser and Miiller, 195 1; Helbig, 1965) seems to indicate that local variations in intensity were high and differences due to variation in exposure not very pronounced. Furthermore, drainage and associated valley asymmetries like those of the Kirchlemer Bach (Fig. 4B), or the larger northeast to east running streams (Fig. 3), are clearly not a result of preferential lateral erosion due to
R. Wende /Geomorphology 14 (1995) 255-265
favourable exposure to sunlight since their steeper and shorter valley sides face in northerly directions. Also the changing location of the larger part of the drainage area of the Gr. Vils and Rott Rivers in a downstream direction from the northern side of the channel to southern side (Fig. 3) contradicts the idea that prevailing northwesterly to westerly winds had considerable influence on their drainage evolution. If drainage asymmetry is assumed to be the result of a lateral shift of the main channel to one side of the basin then streams like the Rott upstreamof the junction with the Bina (Fig. 3) would have shifted some 5 to 6 km to one side without leaving any evidence for this evolution, except for the asymmetry of the drainage itself. Any variations, in microclimate powerful enough to trigger a process like this should have resulted in a similar shift of the Kollbach, located just a few kilometers northeast of the Rott. The same line of reasoning can be applied to question an assumed “climatically” triggered differential headward extension of the tributaries instead of a lateral shift of the main channel. As neither lithological variability and geological structure, nor variations in microclimate appear to be plausible explanations for drainage asymmetries in the study area, the cause of the asymmetry especially of the larger valleys remains questionable. An alternative explanation is thus required. In the Isa-Inn Hill Country, the differences in slope dimensions are closely related to the arrangement of the drainage networks. In valleys where the slopes extend from the valley floor to the watershed, the slope height and slope length is directly restricted by the distance between the valley floor and adjacent watershed. In valleys of higher order the slopes end frequently on isolated hills along interior watersheds. The profiles are located in the inter-basin areas between adjacent tributary basins and are the lower parts of interfluves. Slope height and slope length are therefore determined by the profiles of the latter. The interfluves themselves are functionally related to the adjacent valleys, and in fluvially dissected regions their longitudinal profiles are an approximate inverted mirror image of the valley profiles (Ahnert, 1984). Since opposite tributary basins in the study area have approximately the same relief but a different length on either side of the parent stream, the logarithmic profiles of the valleys and the related profiles of the interfluves are steeper on that side of the parent stream where the distance to the
263
Fig. 6. Asymmetric valley development assuming: (A) a lateral shift of the valley axis, (B) asymmetric extension of tributary valleys on opposite sides of the main valley. Resulting tributary valley profiles (dashed lines) and interlluve profiles (solid lines) have different gradients on opposite sides of the main valley. Interfluve profiles coincide with the slopes of the main valley.
watershed is shorter (Fig. 5). Furthermore, the slopes of the larger valleys are laterally dissected into alternating spurs and hollows, or interfluves and valleys, and as a consequence the valley sides with the steeper interfluve profiles also have the steeper slope profiles. In other words, shorter and therefore steeper tributary valleys have steeper intefluves associated with them. Therefore, a differential extension of the tributary valleys on either side of the main valley generates an asymmetric main valley (Fig. 6B). Drainage and main valley asymmetry are not only linked through a functional relation between interfluves and adjacent valleys, but also through the influence of the tributary stream on the lateral position of the main stream. In catchments or valleys with closely spaced tributaries which are relatively large in respect with the parent stream, drainage asymmetry is a simple explanation for the maintenance of valley asymmetries. Melton ( 1960) found that the position and activity of the channel in respect to the valley-side slopes is one direct cause of valley asymmetry. In many valleys of the IsarInn Hill Country the principal stream receives longer and more numerous tributaries from one valley side. Alluvial fans at the mouths of larger tributaries, the position of the larger channels at the base of the opposite slope, and the frequent local steepening of this side of the valley indicate that the entering tributaries frequently divert the main stream to the opposite valley
264
R. Wende / Geomorphology 14 (I 995) 255-265
side. Indeed, recent earth slides on the steepened lower slope segments are evidence of the activity of basal erosion under present conditions. The characteristic imbalance of feeder streams in asymmetric drainage networks probably causes a diversion of the main channel to the opposite valley side causing a series of locally steepened slopes there. A similar relation between drainage and valley asymmetry was suggested by Siilch (1918). Drainage asymmetry causing differences in slope dimensions and an imbalance in run-off and sediment supply on opposite valley sides appears to be a logical explanation for the lack of symmetry of valleys in the Isar-Inn Hill Country. However, the question as to what causes drainage pattern asymmetry remains. A major factor influencing drainage patterns is the spatial arrangement of the drainage networks. Where this arrangement involves evolution of drainage in certain competitive situations, asymmetric drainage can be produced. Four hypotheses are suggested as possible causes for the drainage asymmetries in the study area: (1) The position of a channel relative to adjacent parallel or subparallel drainage lines restricts the extension of the catchment, particularly in the direction of its closest neighbour, and may therefore result in drainage asymmetry (Fig. 7A). A special case of this situation is the differential tributary development near the junction of subparallel basin as described by Flint (1980). ( 2) Different rates of headward erosion of tributaries on either side of an inter-stream divide, caused by different rates of downcutting of their parallel or subparallel parent streams, may lead to a gradual shift of the dividing watershed towards the minor channel (Fig. 7B). The position of the watershed of the upper Rott is very likely the result of drainage area losses to the adjacent Isen, which has a considerable erosional advantage. Other drainage networks in the study area indicate the same process (Figs. 3 and 2C). Eventually different rates of downcutting lead to capturing which can cause not only drainage but also valley asymmetry (Kennedy, 1976). The sudden change of direction and its associated valley asymmetry along the Gassauer Mtihlbach appears to be the product of stream capture since there is no indication for a lithological or structural control (Fig. 4A). The apparently preferred extension of the smaller north-south trending basins in westerly directions
Fig. 7. Competitive network development and resulting valley asymmetry. (A) Different spacing of parallel/subparallel major drainage lines. (B) Different rates of downcutting. (C) Development of tributaries oblique to an initial terrain slope (modified after Horton, 1945).
could be the result of two further special forms of drainage development in competitive situations: (3) Horton ( 1945) argued that on a surface with low relief and an initial slope, the tributary of a consequent parent stream that runs oblique to the original terrain slope will extend its catchment further to the upslope side than to the downslope side because of enhanced catchment areas and gradients in this direction. The enlargement of the basin and the development of a new generation of tributaries on the valley sides of the parent valley could therefore result in an asymmetric drainage network, with more numerous and longer tributaries upslope of the parent stream (Fig. 7C). (4) Tilting of an existing landsurface could also cause preferential headward erosion of consequent running tributaries and finally drainage asymmetry (Siilch, 1918)) particularly in the Isar-Inn Hill Country (Krebs 1937). However, there is no direct evidence of tilting, although the Pliocene uplift of the Alpine Foreland might have resulted in an initial terrain slope dipping northeast to east for the area of the Isar-Inn Hill Country and could have possibly provided the conditions necessary for the development of asymmetric drainage networks.
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7. Conclusion The differences of’ slope dimensions of the asymmetric valleys in the Isar-Inn Hill Country, and the striking coincidence with the drainage asymmetry, supports the hypothesis that drainage development is a major cause of valley asymmetries. The link between drainage and valley asymmetry in densely fluvially dissected landscapes is the functional relationship between valleys and a.djacent interlluves. The slopes of larger valleys are dissected into alternating valleys and interlluves and are, therefore, related to the characteristics of the drainage network. Furthermore, asymmetric drainage causes asymmetric sediment yield from opposite valley sides and may result in preferential lateral erosion on one valley side. There are several possible causes for drainage asymmetry: the position of an initial channel to its adjacent parallel or subparallel drainage lines; the different rates of headward erosion of tributaries on either side of an inter-stream divide; drainage development oblique to an initial terrain slope; and tilting of a land surface and resulting preferential headward erosion of consequent running streams. The contemporary drainage of densely fluvially dissected landscapes with a long denudational history, is the likely result of an evolution in competitive situations, but due to the complexity of this evolution, it is certainly difficult and often impossible to reconstruct individual evolutions. Nevertheless, any study on valley asymmetry in these landscapes should investigate the relation between drainage evolution and valley development before hypothesised palaeomicroclimatic differences are considered as a means of explaining valley asymmetries.
Acknowledgements The author is especially grateful to F. Ahnert, W. Romer and G. Nans~on for valuable comments. The comments of B.A. Kennedy and an anonymous reviewer helped to improve the manuscript. D. Martin, University of Wollongong, produced the final diagrams. This study was prepared at the Department of Geography, RWTH Aachen, Germany. References Abrahams, A.D., 1984. Ch,mnel networks: a geomorphological perspective. Water Resour. Res., 20: 161-168.
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