Dating fluvial processes from historical data and artifacts

CATENA ELSEVIER

Catena 31 (1998) 283-304

Dating fluvial processes from historical data and artifacts 1 Stanley W. Trimble Department of Geography, University of California, Los Angeles, CA 90024-1524, USA

Received 1 January 1997; revised 16 June 1997; accepted 4 July 1997

Abstract Geographers have long used historical data and artifacts to reconstruct past landscapes. Many of these same data can provide powerful tools for dating stream processes over the past century or so but applications can range from months to millennia. Historical techniques are important not only to mainstream geomorphological investigations but also to fluvial applications in environmental management. The approach is useful for human-induced fluvial changes as well as for those occurring naturally. This paper is intended as an introduction to several of the primary techniques. © 1998 Elsevier Science B.V. Keywords: Fluvial geomorphology;Historical methods; Archeology; Historical data

I. Introduction Historical data and artifacts have always been a primary research approach among cultural-historical geographers. Similar approaches are becoming increasingly important in ascertaining changes of the physical environment. A m o n g the reasons for this recognition are that (1) understanding processes in historical time gives important insights to processes in geologic time and (2) the knowledge of processes and trends of the past century or so allows more effective environmental management. The outstanding work o f Hooke and Kain (1982), now a classic, gives an overview and keen critique and analysis of this approach, especially the use of British sources. Note that historical data as used here (and in other papers cited) refer to cultural data and artifacts. Although

i An earlier version of this paper was presented during the COMTAG International Symposiumon "Time, Frequency and Dating in Geomorphology", held in Tatransk[i Lomnica, SIovakia, June 16-21, 1992. 0341-8162/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0341-8162(97)00042-8

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more natural or scientific dating methods such as dendrochronology, anthropogenic pedogenesis, stratigraphy, and isotopic dating are touched on, full coverage is not appropriate here. The use of historical data in geomorphology goes back at least to George Perkins Marsh (1864). Although normally used to illuminate the last century or so, data and landscape artifacts from classical civilizations were used by Vita-Finzi (1969) in a seminal work to date stream processes over a much longer period, a time scale perhaps archeological rather than historical. Although historical data are most often used to study human-induced geomorphic changes, they sometimes may be used to measure changes which may be occurring quite naturally (Alexander and Nunnally, 1972; Schmudde, 1963). For the United States, a time scale of about 3 centuries, Trimble and Cooke (1991) have amassed and critiqued historical data sources for geomorphic change.

2. Some primary techniques This paper presents some applications of historical data and analysis to the study of fluvial forms and processes. Several specific approaches or techniques are presented with examples. Because such techniques are often used in combination, the paper culminates with an historical site reconstruction which demonstrates the symbiosis of several approaches. For other examples and data sources, the reader is directed to Thomes and Brunsden (1977), Hooke and Kain (1982), Gregory (1979, 1987), Cooke and Doornkamp (1991), Trimble and Cooke (1991) and references given therein. A perusal of examples and sometimes, of the sources themselves, can often give insight and inspiration for a particular application. The following categories have been generalized for this short review format. Most of them dealing with geomorphic effects (form and process), but the last two (climate and land use) deal with causes. However, note that form and process of effects in one place may be the cause of form and/or process elsewhere.

2.1. Use of bridges and bridge plans Highway and railroad bridges are common landscape features. Inspection of older bridges by a practiced eye can often yield immediate information about stream processes. An aggrading stream is often indicated by reduced stream openings and by the burial of structural members (e.g., wingwalls) which are usually exposed to the stream. Degrading streams, on the other hand, can often be diagnosed by old water lines left on structural members, by exceptionally large openings, and by the exposure of structural members (e.g., footings, pilings) which are usually placed beneath the surface. Williams (1978) used the remains of an old bridge to show changes of channel form on the North Platte River in Nebraska. Much more valuable to the investigator is the availability of bridge plans. Almost always, these will include a stream and valley cross-section surveyed before bridge construction which can be resurveyed for comparison (Fig. 1). When comparing profiles,

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one must ensure that the present bridge has not induced local scour or deposition which would thus invalidate comparison. Some plans also include a detailed topographic map of the stream reach and some may also include surveyed cross-sections at some distance upstream or downstream. The latter are particularly valuable because they tend to be free of scour effects induced by some bridges. In many locations, a succession of bridges has been located at the same site. Highway bridge plans often go back to the turn of the century and railroad plans to the mid-late 19th century (Happ et al., 1940), and the plans of the earlier, now-defunct bridges may be far more valuable. Fortunately, the same datum is usually used for successive bridges at a site so that resurveys of century-old profiles are facilitated. Even when datum changes, parts of the old bridge may be excavated to reestablish elevations. In some instances, government agencies inspect bridges at some frequency. Inspection reports usually contain considerable description and measurements of site conditions, and often include photographs which allow time-lapse photography. 2.2. Dams, mills and reservoirs Dams, water-powered mills, and reservoirs have an obviously intimate relationship with streams. Disequilibria in streams often create severe problems in the operations of mills and reservoirs and such problems may have been documented or can be established. Trimble (1970b) used mill dams, in part, to document the aggradation and degradation of streams on the Georgia Piedmont. An example is Maudlin Mill in Hall County, GA (Fig. 2, Plate 1). The dam was built in 1865 to the bank height of 3.5 m, creating a channel reservoir. Dams are always attached to bedrock wherever possible and so was this dam. Since cofferdams were rarely used for such locally-built structures, it may be safely deduced that the stream was flowing over bedrock at the time of construction. Further evidence of that assumption was given by an 1879 water power survey (U.S. Census, 1879) that the hydraulic head, the vertical distance from pond level (usually top of dam) to tailwater was 3.5 m, the height of the dam above bedrock.

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As a result of poor land use, aggradation began in the late 19th century. An eye-witness in 1910 reported that the head had been reduced to about 2 m, indicating the stream was aggrading (Trimble, 1970b). By 1935, an eye-witness reported that a probe indicated that the uppermost part of the dam was 1 m below the streambed (Trimble, 1970b), meaning that 4.5 m of aggradation had occurred between 1865 and 1935. Later in the 1950s, and as a result of improved land management, the stream began to degrade (Trimble, 1970b). By 1969, about 0.7 m of the then-deteriorated wooden dam had been exposed by stream degradation (Fig. 2, Hate 1). The remnants of the dam now serve as a benchmark to measure future stream evolution. Other types of low dams, such as weirs and irrigation diversions, are similarly useful to gauge stream changes (Gilbert, 1917; Thornthwaite et al., 1942; Vita-Finzi, 1969; Cooke and Reeves, 1976; Thoms and Walker, 1993). Although nearly all water-powered mills had reservoirs, most of these were channeltype reservoirs which had very low trap efficiency for sediment, However, some mills and later hydroelectric and flood-control dams have a large volumetric capacity in relation to their drainage area and thus have a high trap efficiency so that sediment yield can be measured (Trimhle and Bube, 1990). Data from such large reservoirs may be useful on a decadal time scale (Eakin, 1936; Trimble and Lurid, 1982; Foster et al., 1990; Labadz et ai., 1991; Trimble and Carey, 1992; McManus and Duck, 1993; Foster and Walling, 1994; Foster, 1995, 1996; Fig. 3). Most reservoirs are given continuing surveys. These data must often be ferreted from repositories, but compendia are periodically published, usually by governmental agencies (e.g., Dendy and Champion, 1978). Forty years of such data from a large reservoir system, the Tennessee Valley Authority, were used by Trimble and Bube (1990) to construct trap efficiencies for sediment coming directly into reservoirs and also for sediment which had already passed through upstream reservoirs (Fig. 3). Using these curves, the data were used to establish long-term rates of sediment accumulation, yields and fluxes in a stream basin at 113,000 km 2.

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Plate 1. Mauldin milldam, exposedby streamdegradation(Trimble, 1974). 2.3. Roads and causeways

Like bridges, roads (including railroads) and causeways serve as benchmarks to measure changes of stream morphology and process. An example is from Coon Creek in the Driftless Area of Wisconsin (Fig. 4). The 1853 surface is the pre-agricultural soil as determined from borings (McKelvey, 1939). McKelvey (1939), Happ et al. (1940) and Happ (1944) dated the pre-agricultural soil of the Driftless Area by cultural artifacts, showing that the overlying sediment was of historical origin. This was 3 decades before Knox (1972) used isotopic dating, apparently unaware of the earlier work by McKelvey, Happ and others. The 1904 surface is the level of the floodplain where a railroad causeway was built. Causeways are built of upland materials with more strength than floodplain soils. The differences in texture and usually color of these upland materials

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create a definite contact with the existing floodplain. [It has been suggested that causeways are created by excavating the floodplain to either side of the right-of-way but alluvial soils are often too weak to support trains or other heavy vehicles. Moreover, Coon Creek Profile CV30 Vernon County, W i s .

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using alluvial soils would require lifting materials rather than moving them laterally, an important consideration before modern earth moving equipment.] The 1930 surface in Fig. 4 is a roadway abandoned when the bridge was relocated 100 m downstream. This roadway was described by an eye-witness as being maintained level with the floodplain until the time of abandonment in 1930. It was located by borings, the diagnostic feature being crushed limestone gravel at a uniform level. Note that the rate of floodplain aggradation increased significantly from the first period (51 years) to the second period (26 years). This rate increased even more with 1.2 m between 1930 and 1938. The 1938 profile was surveyed just upstream by a governmental agency (McKelvey, 1939) and the 1976 profile was surveyed using the 1938 datum (Trimble and Lund, 1982). These five dated floodplains furnish a remarkable record of the rates of accelerated sedimentation in a disturbed basin. Roads can also be useful for measuring lateral movement of streams. If the former position of a road in relation to the stream can be determined from documentary evidence such as old maps or aerial photographs, its present location will allow an average rate of lateral migration to be calculated. Generally, significant stream migration takes place on the time scale of decades or centuries, but on occasion it occurs in a few years, or even months. The utility of roads as an indicator is greatly enhanced by the availability of aerial photographs. When a stream impinges on an important road, authorities will normally take whatever measures are necessary to protect the road. Usually some sort of documentation is prepared, often with plans and maps. Structures put into place then act as benchmarks to measure future stream movement. In Wisconsin, for example, a roadbank reinforcement installed in 1947 later showed that the cut bank had advanced about 10 m in 5 years (Trimble, 1975a,b). Other road-protection structures useful for future measurement are dikes and levees. Additionally, roads are often raised above normal flooding or aggrading floodplains by fill, but the level of the old road beneath may be ascertained from construction plans and borings. 2.4. Buildings

Except for occasional mills, buildings are rarely constructed on active floodplains, and even when close to streams, they are usually sited on terraces. Thus, when a structure is affected by sediment, it indicates important changes of stream regime. Generally, when a building is significantly impacted by water a n d / o r sediment, steps are taken to move or raise the building if possible. If that is not possible, the building is usually dismantled, leaving only the foundation, which itself can serve as a benchmark. Once the foundation has been covered with sediment, however, the location must be established from old maps, land plats or perhaps eye-witness testimony. Likewise, the chronology must be established from such sources as maps and land survey plats, tax records, or eye-witness accounts. Unlike roads and causeways which may be located by borings, it is best to excavate around as much of the building as possible because it is necessary to see how the building's occupants interfaced with the stream. For example, did an entrance face the stream? Artifacts between the building and the stream such as steps, walks, fences (McKelvey, 1939) and small outbuildings would imply that the area

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was frequented by people at one time, thus implying low frequency of flooding. In studying the buried village of Village Creek, Allamakee County, IA, for example, Trimble found a buried chicken house between a dwelling house and the stream, The work of archaeologists relating to buildings is highly instructive (Butzer, 1964; Butzer and Hansen, 1968; Vita-Finzi, 1973, 1978). In Ethiopia, Butzer (1971) used a ruined fort to gauge long-term lake fluctuations. In parts of Europe (e.g., Germany), older buildings have historic flood levels marked, so that high flood discharges could be calculated and a flood series reconstructed. Under certain circumstances, buildings may be useful for measuring stream channel erosion. The work of Womack and Schumm (1977), Cooke and Reeves (1976) and others in studying arroyos (gullies) in the western United States has much applicability here and deserves detailed study. 2.5. Instrumented topographic surveys These are the best baseline data to be obtained, but they are not to be found for all locations. Already discussed is topography associated with bridges, and the utility of earlier surveyed cross-sections is shown by the 1938 profile in Fig. 4. Other baseline sources are governmental river and estuary surveys (Gottschalk, 1945; Dolan and Bosserrnan, 1972). Where they are to be found, Vigil Network data are extremely useful because they are developed by geomorphologists and are designed for restudy (Emmett and Hadley, 1968; Leopold and Emmett, 1965; Osterkamp et al., 1990). The U.S. Army Corps of Engineers (COE) has surveyed longitudinal and cross-sectional profiles on many U.S. rivers over the last century or so. These were used to good advantage by Adler (1980) and James (1989) to study the movement of erosional debris from hydraulic mining in California. Kondolf and Curry (1986) used COE profiles to show channel migration while Kesel et al. (1992) used them to establish a sediment budget for the lower Mississippi River. When possible in using old surveys, one should always check to see if level lines are closed, and if so, to what level of precision. Measuring increments of centimeters across a broad floodplain may be inaccurate unless 3rd- or 4th-order survey standards are maintained. Historically, transits or theodolites have not been capable of that level of precision. Included in this category is the use of historical stage-discharge rating records for stream gauging stations. For the United States, some of these were begun in the 19th century and were sometimes revised frequently. Each revision should entail at least one surveyed cross-section which may be contained in the records. These have been used by Cooke and Reeves (1976) and Williams and Wolman (1984) to demonstrate channel changes. Additionally, gauging stations sometimes include control structures, such as weirs, which can sometimes be used for dating (Trimble, 1975a,b). Although at a lower resolution, one could consider old topographic maps in this category. Butzer (1971) used maps and travel accounts to document lake levels and delta growth (1888-1930), in Lake Rudolf, Ethiopia. Older topographic maps require many caveats but are still useful (Hooke and Kain, 1982; Lawler, 1993; Downward, 1995). Graf (1983a,b) for example, was able to show channel migration of the Salt River near Phoenix, Arizona from 1868 onward. Erskine et al. (1992) documented channel cutoff

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from 1879 in S.E. Australia, while Gregory and Ovenden (1980) demonstrated increases of drainage density in the Southern Pennines. Soils maps dating to the 19th century are often at a larger scale and include many features of interest to the geomorphologist, perhaps because the training of soil scientists was quite similar to that of physical geographers (Trimble, 1970a). 2.6. Land (cadastral) surveys

Although given in only two dimensions, land surveys can often supply important information to the fluvial geomorphologist. In the United States, the original plats (detailed maps) of survey have been used to establish pre-agricultural floodplain conditions (Trimble, 1970a,b), upland vegetation (Trewartha, 1940; Knox, 1977) and stream widths (Cooke and Reeves, 1976; Knox, 1977; Eschner et al., 1981). In California, Kondolf and Curry (1986) used boundary surveys of Spanish land grants to estimate channel changes. In other areas and more recent periods, ongoing land surveys sometimes give useful descriptions of geomorphological interest. 2.7. Aerial photographs

In the United States, the general coverage of stereographic aerial photography dates from 1937-1938, although there is limited coverage from ca. 1925 (Trimble and Cooke, 1991). Coverage in the UK generally dates from the mid-1940s (Hooke and Kain, 1982). The utility of aerial photography is a function of scale, photographic quality, and the availability of stereographic coverage. With good vertical photographs, Hoag (1983) was able to use standard photogrammetric methods to quantify channel erosion in Orange County, California from 1938 to 1983. Graf (1975) was able to document the rapid expansion of urbanization in Colorado which caused severe channel disruption. In Wisconsin, significant decreases of stream response have been inferred from striking decreases in drainage density as measured from air photos (Trimble and Lund, 1982; Fraczek, 1987). Conversely, tidal creek channel networks were shown to be expanding after 1943 in northern Australia (Knighton et al., 1992). Air photos are extremely useful in establishing rates of channel migration and channel change (Schumm and Lichty, 1963; Graf, 1978, 1982; Williams, 1978; Kondolf and Curry, 1986; Lawler, 1993). Aggradational processes are difficult to detect but some attendant effects such as the creation of backswamps can be seen and measured. 2.8. Ground-based oblique photography

While not systematically available like aerial photography, ground-based photography can date back to the mid-19th century (Hooke and Kain, 1982) and generally offers excellent scale. Some major sources for the United States are given by Trimble and Cooke (1991). The most valuable coverage is sequential photos at the same place, known as time-lapse photography (Malde, 1973; Rogers et al., 1984). Graf (1979a) has used such imagery to show the development of channels in montane valleys, (1982) arroyo filling and vegetational stabilization, (1983) changes of bed material in the Salt

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Plate 2. Downstream view of Ratz gully, Winona County, MN, 1940. The hanging fence marks the 1933 cross-section. Using the height of the figure standing beneath the fence (1.8 m, barely visible in this photographic prin0, oblique photogrammetrywas used to reconstruct the gully in 1940 (Fig. 5). In the absence of the figure, the car parked along the fence could have been used.

River, Arizona, and (1994) stabilization by vegetation of sediment deposits along the Rio Grande River. Trimble and Lund (1982) showed the formation and healing of hillsides gullies in Minnesota and also showed the transformation of tributaries from sediment sinks to sediment sources as stream response increased. Williams (1978) and Eschner et al. (1981) have also used this imagery to show changes of channel size and form for the Platte and North Platte Rivers in the central U.S., while Williams and Wolman (1984) demonstrated channel scour downstream of dams. As with vertical aerial photography, photogrammetric techniques can also be used with oblique photography making it possible to make precise measurements in some cases (Graf, 1979b). Oblique aerial photography is also available for some areas (Trimble and Cooke, 1991). An example using both repeat photography and photogrammetry is shown for a gully which formed in a Pleistocene terrace above the Whitewater River valley (Minnesota) in the early 20th century (Plates 2 and 3, Fig. 5). Plate 2 shows the gully in 1940 with an undermined fenceline, built only 7 years previously, suspended above the gully. Using the size of the car, the dimensions of both channels can be constructed (Fig. 5). Plate 3 is a recent repeat of Plate 2. While photogrammetry could be used to draw a cross-section from this photo, it was easier to survey the profile (Fig. 5). Thus, the gully began in the early 20th century as did many such gullies in the region, expanded rapidly after 1930, and continued to expand after 1940. However, the partially buried trees now growing on the aggraded valley floor are 40-45 years old,

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Plate 3. Repeat of Photo 2 site, made in 1988.

meaning that the gully was stabilized and even filling by the mid or late 1950s. The process now is slow aggradation of the valley floor. The earlier history is inferred from stratigraphy. The dark Mollisols (Fig. 5) were at the surface at the time agriculture began (McKelvey, 1939; Happ et al., 1940). The

Ratz Gully Cross-Section Whitewater River Basin SW 1/4 Sec 3, T108N R10W Winona County, Minnesota 1850-1979

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historical sediment above it accumulated from local upland erosion before the valley gullied. In many cases, such valley floors were ditched for drainage, thus concentrating flows and causing the valley to gully. Cooke and Reeves (1976) found a similar process in the southwestern U.S. Excellent accounts of gully growth using several appropriate techniques are found in the work of Ireland et al. (1939). 2.9. Travel accounts and other contemporary descriptions

The utility of travel accounts takes two basic forms. The first is a description of past events or changes, while the second is the description of contemporary or baseline conditions useful for later comparisons. In all cases, one must consider (1) the scientific credentials of the observer, and (2) the stage of scientific development at time of observation. In the latter instance, an example of an erroneous observation would be a 19th century observer attributing erratic rocks to the water currents of the biblical flood. For an example of contemporary change, one can do no better than the famed English geologist, Sir Charles Lyell, who observed the early fluvial consequences of agricultural settlement on the Piedmont of Georgia: " S o late as 1841, a resident there could distinguish on which of the two branches of the Altamaha [River], the Oconee or Ocmulgee, a freshet [flood] had occurred, for the lands in the upper country drained by one of these (the Oconee) had already been partially cleared and cultivated so that that tributary sent down a copious supply of red mud, while the other (the Ocmulgee) remained clear, though swollen" (Lyell, 1849). An example of a baseline scientific observation comes from the American surveyor and naturalist, Owen (1847). Before extensive agricultural settlement in the upper midwestern U.S., he could observe fish and bottom formations in streams up to several meters deep. Of course, this would have been impossible at any time since agriculture began. Accounts from scientific observers such as Lyell and Owen tend to be dependable and are often extremely helpful. Observations from less-qualified people also can be helpful, but may need more qualification or interpretation (Hooke and Kain, 1982). Trimble and Cooke (1991) give many useful sources for travel accounts in the United States. Related sources are contemporary newspapers, periodicals, books, government records and unpublished manuscripts. For example, Kondolf and Curry (1986) used newspaper accounts to study channel migration in California, while Knox (1987) used them to establish historical floods in Wisconsin. Trimble (1974) used newspapers and periodicals to trace the progress of soil erosion on the Southern Piedmont. 2.10. Mobile cultural debris

Cultural artifacts useful for dating and considered earlier are those fixed in place. The present category considers mobile artifacts such as bottles, cans, package wrappers and any other datable artifacts often found in cut banks or by intentional excavation such as

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trenches dug to examine stratification. Although an artifact may be precisely dated in some cases, its location can only give an earliest possible date. For example, a 1952 automobile license plate might have been removed from the vehicle in 1952, stored in a garage for 2 years, dumped into a stream in 1954 where it was buffed in a point bar in 1955. The point bar may have eroded away in 1962 and the plate buffed 20 cm deep on a downstream floodplain. The only allowable conclusion is that there has been a minimum accretion of 20 cm at that location since 1952. However, finding several items at similar levels with similar dates might allow a stronger inference. For example, an intact dump located in a floodplain with several items of similar dates could be valuable (Costa, 1975). A related, and perhaps far more effective way of tracing fluvial sediments and surfaces is by use of anthropogenically-produced materials such as radionuclides, heavy metals and other wastes (e.g., Davies and Lewin, 1974; Lewin et al., 1977; Wolfenden and Lewin, 1978; Bradley, 1982; Macklin, 1985; Knox, 1987; Marron, 1989; Walling et al., 1992; see the excellent reviews in the work of Graf, 1994 and Foster and Charlesworth, 1996). Methods of dealing with these tracers are beyond the scope of this paper. 2.1 I. Stream and sediment discharge records

Stream discharge data go well back into the 19th century in the U.S. are quite plentiful for this century, and most are easily available (Trimble and Cooke, 1991). Data for the UK have neither the density nor longevity of the U.S. but have improved greatly since about 1950 (Hooke and Kain, 1982). The flood record for Britain has been lengthened greatly by the Institute of Hydrology using historical techniques (Hooke and Kain, 1982) and at least two other excellent guides to historical streamflow have appeared (Jones et al., 1984; Sutcliffe, 1989). In the Upper Mississippi River, Knox et aI. (1975) were able to construct the annual flood series for the period after ca. 1850 showing an amelioration around the turn of the century. In the southeastern U.S., Trimble et al. (1987) used streamflow records dating from 1900 to show that reforestation had decreased streamflow for ten large basins. Potter (1991) showed that annual floods had decreased in southwestern Wisconsin after 1940 and suggested that land treatment (rather than land use) accounted for the change. Sediment discharge records in the U.S. date from the early 20th century and both quantity and quality of sampling has increased (Trimble and Cooke, 1991). These records were used by Meade and Trimble (1974) to show the decrease of sediment yield along the eastern seaboard of the U.S. between 1910 and 1970 as a result of (a) better land use, and (b) reservoir construction (see also Meade, 1982; Meade and Parker, 1985). Trimble (1975b), Trimble (1977) compared these same sediment yields to upland erosion rates based on historical soil profile truncations to show that streams were not in steady state. Hadley (1974) demonstrated decreases of sediment yield on the Colorado River 1926-1960 and related them to decreases of grazing. Trimble and Carey (1984) showed how historical sediment concentrations could be calculated from reservoir deposition rates and unit runoff data.

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2.12. Climate

The primary interest here is recorded climatic records, especially regular records kept by governmental agencies. However, Hooke and Kain (1982) list private diarists going back to 1337 who periodically recorded British weather. Lamb (1977, 1982, 1988) has been resourceful and ingenious in reconstructing past climates from highly varied historical sources including official records when possible. His synthesis of these techniques serves as a model. The UK is fortunate in having much of the climate over the past 2 centuries or so synthesized and more accessible to use (e.g., Lamb, 1972; Kington, 1976; Lawler, 1987). Official climate records sometimes go back to the mid-19th century in the U.S. and Europe, but scattered records were collected earlier. For example, Cooke and Reeves (1976), in their far-reaching analysis of arroyos in the southwestern U.S., were able to use early climate records from U.S. Army posts. In the upper Mississippi Valley, Trimble and Lund used official records to analyze precipitation trends back to 1865, showing that climate was distinctly out of phase with the hydrologic and geomorphic process shown to be occurring. An exemplary recent study by Rumsby and Macklin (1994) correlates climate and channel changes from 1700 to the present in the River Tyne, UK. Perhaps few areas of the world have received the attention lavished on the climate of the southwestern U.S. in explaining the historical formation or demise of arroyos (Graf, 1983b). Among the aspects of climate examined have been long-term annual trends, seasonality, frequency, intensity, and more recently, connections with the Southwestern Monsoon and El Nifio-Southern Oscillation events (e.g., Bryan, 1925, 1928; Thornthwaite et al., 1942; Leopold, 1951; Cooke and Reeves, 1976; Hereford, 1984; Bailing and Wells, 1990; Webb and Betancourt, 1990; Hereford and Webb, 1992; Betancourt and Turner, 1993; Hereford, 1993). 2.13. Land use

Along with climate, land use helps drive many geomorphic changes. Thus, reconstructing land use becomes critical to many studies. Hooke and Kain (1982) give excellent coverage to the use of tithe, topographic, county and UK Land Utilization Survey maps, some of which go back to the early 1700s. Trewartha (1940) and Knox (1977) used original plats of survey to establish primeval vegetation in southwestern Wisconsin, while Trimble (1970a) used them to study early floodplain conditions in Georgia. Graf (1979a) used photography to reconstruct land use for a basin in Colorado, 1859-1974. He then related land use changes to hydrologic and geomorphic changes, specifically channel incision. In the perennial and ongoing attempt to explain arroyos in the Southwestern U.S., Denevan (1967) and Cooke and Reeves (1976) splendidly reconstructed livestock by census reports, travel accounts and other sources. Both studies concluded that animals were important but only one of many causal factors. Where data are more complete, such as those of census reports, land use reconstructions should be as precise as possible because (a) increasingly sophisticated information

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about the hydrologic and geomorphic effects of different land uses and treatments is available, a n d / o r (b) we may be able to correlate historical land use with contemporaneous geomorphic phenomena in model building. An example of the latter is a study by Graf (1979a) which related land use to channel incision. Another example is the study of reforestation and water yield decreases in the Southern Piedmont by Trimble et al. (1987). Their long-term results of 10 large basins, together with available short-term experimental data, permitted a model which explains 50% of water yield variance in humid areas as a result of reforestation or deforestation and is presently the standard stochastic model for water yield changes (Maidment, 1993). Potter (1991) was able to show a decrease in the annual flood series of the Pecatonica River in southeastern Wisconsin even though land use held relatively constant. He attributed the amelioration of flooding to improvement of land treatment and decreases of drainage density. Historical soil erosion and associated hydrologic changes have been analyzed using detailed land use reconstructions from census data in the U.S. (Trimble, 1974) and Canada (Wilson, 1989). Trimble and Lund (1982) were able to show a lag of hydrologic and geomorphic processes so that there was a hysteretic relationship between land use and the dependent variables of erosion and sedimentation. The lag was attributed to changes in soil condition. The use of agricultural census data is complex and confusing because categories and definitions often change from one census to the next. For the U.S. census, at least, county enumerations are for 'land in farms' only and sometimes cover only fractional parts of counties. Where available, the census manuscripts, rather than the published reports, give far more detailed information (Conzen, 1969). Another major problem is that areas of enumeration units change with time so that the boundaries and areas must also be reconstructed (Trimble, 1974; Appendix F). Unfortunately, there are as yet no guides to the use of census data in reconstructing historic land use and a great need clearly exists. There is often a need to extend land use data beyond the effective dates of the census. This can sometimes be done effectively by surrogates. Thus, historic sheep populations were estimated from the weight of wool clipped each year (Noble and Tongway, 1986) and erosive land use was extended back in time by calibrating from densities of slave and non-slave populations (Trimble, 1974). Such surrogate retrodictions might be used for other historical data, but must always be used with caveats.

3. Integrated site example Chaseburg, Wisconsin, was founded in the 1850s on a series of low terraces formed on and around an island in Coon Creek (Fig. 6). The early economic base was a mill made possible by the bedrock shoals at the location. The village prospered and, according to available photographs, had many houses and commercial buildings by the late 19th century. Beginning early in the 20th century, flooding and aggradation began as the result of poor land use upstream. By the 1940s, most of the original village had been covered by historical sediment. Coon Creek had aggraded about 3 m and flooding

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298 Well corner Co. guage

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Fig. 6. Burial of Chaseburg, Vernon County, WI. Map of Village (inset), greatly simplified.

was frequent. Most buildings had been raised, moved, or dismantled. By the 1970s there was little visible evidence that anything unusual had happened and many local residents knew nothing of the buried parts of the village. The primary key into the village's past was a milldam inspection report done by the state of Wisconsin in 1914. It featured an instrumented survey which fixed the relative elevations of the stream, floodplains, roads, and various terrace surfaces. A subsequent survey in 1933, using the earlier datum, placed an elevation on a newly-constructed concrete milldam. Photos contained in the file showed the rapid burial of the milldam from 1933 to 1946. Excavation of that milldam with its elevation ('H', Fig. 6, Plate 4) allowed a comparison of the elevations of 1914 with those of 1977. Another key was obtaining an eye-witness account of the rapid burial of a house ('B', Fig. 6). Built in 1903 on a low terrace, the house was inundated in 1907 by a flood described as the highest previous to that time. The owner then raised the house to the 1907 flood level by jacking up the building and adding 1.3 m to the height of the foundation. As the stream aggraded, the house became untenable and was dismantled in the late 1920s. Aerial photography taken in 1934 showed no trace of the house. Excavation of the foundation in 1977 revealed the 1907 flood level as well as the Mollisol of the terrace surface on which the house was sited. The presence of such a well-developed soil indicates very low rates of sediment accretion for a century or more before the house was built• That plus the proximity of the house to the stream suggests lateral stability of the channel. Old photographs from various sources including local citizens showed the village as it had been. Among the features noted were the old hotel ('D', Fig. 6) and a village street ('F', Fig. 6). With their location known, it was relatively easy to find them with borings and local excavations giving still more estimates of deposition rates. Another feature in the old photographs was the present County Highway Garage ('A', Fig. 6), which was built in 1915 as an automobile dealership. The old photographs show a steep

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299

Plate 4. Concrete milldam at Chaseburg buried by sediment and excavated in 1977. Survey rod stands on elevation set in 1933.

ramp going up the terrace escarpment from the county road to the entrance. Eye-witness accounts stated that the ramp was so steep that cars had to be backed up the ramp because reverse gear was the lowest ratio in a car at that time. Presently, one looks down from Highway 162 (Fig. 6) into the entrance of the garage although the ground around the garage has been filled about 0.6 m in recent years. According to an atlas of Vernon County (published in 1917), new growth in Chaseburg was taking place on the surrounding high terraces rather than on and around the island. There is additional evidence available for Chaseburg, but this abbreviated account should demonstrate the use and interplay of historical data in researching an aggradational site.

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4. Summary and conclusions Historical data and techniques can provide powerful tools for dating processes over the past few centuries and particularly during the last few decades. Such information is important not only to more traditional theoretical applications, but also to considerations in environmental management. Indeed the importance of the techniques is shown by the fact that Cooke and Doomkamp (1991), (p. 73) have included a section on historical techniques in the newest edition of Geomorphology in Environmental Management, a standard text. While this short paper has touched on most major techniques used by historical geographers, there are many permutations of each not covered here. Geomorphologists would do well to acquaint themselves with the literature and techniques of both historical geography and archaeology for their areas of interest.

Acknowledgements This paper was prepared for the COMTAG Symposium on 'Time, frequency and dating in geomorphology' held in Stara Lesna, Slovakia, in June, 1992. I thank Asher Schick for inviting me to the meeting and I thank the organizing committee which granted me travel funds. Although a selected proceedings of the meeting was not published as planned, the editors, Helmut Brueckner (Marburg) and J.B.J. Harrison (New Mexico Tech) nevertheless did an excellent job and I thank them for recommending this particular paper to Catena for publication. That process also gave me the opportunity to bring the bibliography up to date. I am grateful to many people who taught me historical methods, but especially I thank Louis DeVorsey. Karl Butzer and Claudio Vita-Finzi provided early inspiration.

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