Spatial distribution of channel heads in the Polish Flysch Carpathians

Catena 127 (2015) 240–249

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Spatial distribution of channel heads in the Polish Flysch Carpathians Eliza Płaczkowska ⁎, Marek Górnik, Ewelina Mocior, Barbara Peek, Piotr Potoniec, Bartłomiej Rzonca, Janusz Siwek Institute of Geography and Spatial Management, Jagiellonian University, Gronostajowa Street 7, 30-387 Cracow, Poland

a r t i c l e

i n f o

Article history: Received 29 June 2014 Received in revised form 29 December 2014 Accepted 31 December 2014 Available online xxxx Keywords: Channel head location Headwater channels Physiographic parameters of catchment Flysch Carpathians

a b s t r a c t One of the key topics in modern geomorphology is the identification of locations where the drainage network begins. However, there still exists a gap in the research literature in the area of channel head location in flysch regions characterized by unique hydrogeological conditions. In this study, we have identified the spatial distribution of channel heads and determined threshold values of morphometric parameters for catchments contributing to channel heads in the Polish Flysch Carpathians. We surveyed a total of 401 channel heads on the main ridge of the Połonina Wetlińska Range using GPS. The DEM was used to identify 16 physiographic parameters for each catchment, and statistical correlations between catchment parameters were also calculated. We compared field data versus the drainage network produced using a topographic map and DEM. The channel head density based on field mapping is five times higher than the density estimated using a topographic map, and 250% lower than that obtained from a DEM. Most channel heads are recharged by very small catchment areas. The threshold value for catchment size has been shown to be about 0.01 km2. The mean slope gradient responsible for channel head formation is 18° in the study area. Given the very small size of catchment areas contributing to channel heads in the study area, remote sensing methods are not useful. Fieldwork is still the best method of mapping channel heads. The distribution of channel heads across the Połonina Wetlińska Range in the Polish Flysch Carpathians cannot be described using simple relationships. Correlations between topographic attributes that are common in other physiographic regions are not very clear in flysch areas. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Headwater channels, known as lower order channels according to the Horton–Strahler classification (Strahler, 1957), play an important role in mountain relief formation (Davis, 1899; Horton, 1945; Schumm, 1956; Morisawa, 1957; Strahler, 1957). Headwater channels are key sources of water, mineral matter, and organic matter in the fluvial system (Dietrich and Dunne, 1978; Sidle et al., 2000; Tsuboyama et al., 2000; Gomi et al., 2002). Thus, one of the key topics in modern geomorphology is the identification of locations where the drainage network begins (Montgomery and Dietrich, 1988, 1989, 1992; Hancock and Evans, 2006; Jaeger et al., 2007; Imaizumi et al., 2010; Henkle et al., 2011; Julian et al., 2012). According to Montgomery and Dietrich (1988), a channel head is the part of the slope at the highest elevation featuring concentrated water flow and sediment transport between two distinct channel banks. In effect, a channel head is the upper boundary of fluvial processes, downstream of which one can find perennial, intermittent and ephemeral flow (Dietrich and Dunne, 1993; Henkle et al., 2011). Henkle et al. (2011) identified three types of channel heads: (1) abrupt — one distinct starting point, (2) gradational — no specific location, but ⁎ Corresponding author. E-mail address: [email protected] (E. Płaczkowska).

http://dx.doi.org/10.1016/j.catena.2014.12.033 0341-8162/© 2015 Elsevier B.V. All rights reserved.

development is along several meters of length, (3) channel initiation zone — several channel heads found across an area of b 20 m2. Channel heads are found at the starting places of first-order stream channels (Horton–Strahler classification). First-order channels are predominant in every morphological and climate zone and constitute more than half the total length of river and stream channels (Schumm, 1956; Goudie, 2006, p. 371). In addition, their location on the slope can vary over time, moving upslope due to upstream erosion or downslope when a hollow becomes filled due to slope processes (Bull and Kirkby, 2002). On the other hand, the location of channel heads in different parts of the world may vary based on geological structure, climate conditions, land cover, and human activity, all of which affect the influx and outflow of slope sediments (Montgomery and Dietrich, 1988; Dietrich and Dunne, 1993; Goudie, 2006). In light of the above data, the identification of channel heads and their spatial distribution is a complex and intriguing task. The best way to identify the location of channel heads is field mapping (Montgomery and Dietrich, 1989). However, it is a time-consuming and expensive method, and may not be feasible due to difficult terrain or large study area. Hence, researchers have been attempting for years to estimate threshold values for a variety of channel formation parameters using remote sensing methods in order to avoid fieldwork. The first attempts to identify first order channels were based on topographic maps. Researchers used both subjective methods, e.g. visual

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interpretation of contour line curvature (Krumbein and Shreve, 1970), and quantitative methods based on the angles of contour lines (Morisawa, 1957; Lubowe, 1964). Next, threshold values for slope gradients and channel gradients were identified. These values describe the transition from slope to channel (Shreve, 1974; Tarboton et al., 1991; Church, 2002). However, threshold values are not universal and depend on climate conditions and geological structure, which means that values will vary from region to region (Shreve, 1974; Tarboton et al., 1991). Another parameter used to identify channel heads is the size of the catchment area where the transition from slope processes to fluvial processes takes place. Many researchers define this area as 1 km2 based on observations of hydrogeomorphological processes as well as catchment morphometry (Montgomery and Foufoula-Georgiou, 1993; Gomi et al., 2002; Stock and Dietrich, 2003; May and Gresswell, 2004). Montgomery and Dietrich (1988) noted that as local gradient increases, the catchment area of the channel head decreases. This pattern holds true especially in landslide areas and other steep areas where the location of the channel head depends more on surface parameters than geological structure (Montgomery and Dietrich, 1988, 1989, 1992, 1994). Familiarity with the catchment area and the local gradient is essential in the identification of channel head locations using digital elevation models. Moreover, the size of the given catchment area is a key factor in the shape of the drainage network identified using a DEM. The larger the surface area used for calculations, the larger the part of upper channel sections that is omitted. In effect, the density of the calculated drainage network decreases (O'Callaghan and Mark, 1984; Tarboton et al., 1991). At the same time, Tarboton and Ames (2001) note that there exist different threshold values, depending on the geographic region, for catchments of channel heads. Attempts to identify threshold values of morphometric parameters for channel heads using cartographic materials have not always produced the desired results. Furthermore, the diversity of natural environmental conditions affects threshold values around the world. Most research on channel heads and headwater areas has been conducted in semi-arid, tropical and subtropical regions (Montgomery and Dietrich, 1989; Sidle et al., 2000; Tsuboyama et al., 2000; Hancock and Evans, 2006; McNamara et al., 2006; Imaizumi et al., 2010; Julian et al., 2012). In temperate climate zones, this type of research has been conducted in areas mostly built of crystalline rocks and sometimes sedimentary rocks (Brummer and Montgomery, 2003; Jaeger et al., 2007). Although recently this type of research has been conducted in areas built of sandstone and shale (Imaizumi et al., 2010; Julian et al., 2012), there is still a need to identify channel initiation sites in flysch regions. Flysch is a deep marine clastic rock consisting of a succession of alternating thick layers of sediment ranging from sandstone, shale, marl to clay (Kotlyakov and Komorova, 2007, p. 264). The term “flysch” refers especially to Alpine geology. In recent years, flysch is considered to be turbidites deposited in a deep trench in close proximity to an active plate boundary (Van der Pluijm and Marshak, 2004, p. 17). For this reason, flysch regions are characterized by unique hydrogeological conditions — there is an alternation of permeable and impermeable rock layers. And since these types of rocks are widely distributed across all the continents (Van der Pluijm and Marshak, 2004), it is crucial to recognize the initiation of the channel network also in flysch belts. The purpose of the paper is to identify the spatial distribution of channel heads and the threshold values of morphometric parameters for upslope areas contributing to channel heads in the Polish Flysch Carpathians with the Połonina Wetlińska Range as the example. This particular study area is represented by characteristic flysch sequences of sandstone and shale representative of worldwide flysch regions. The following specific tasks associated with the research are discussed: (1) analysis of channel head location, (2) analysis of the relationship between the morphometric parameters of catchments contributing to channel heads, (3) assessment of the choice of channel head identification method versus the resulting channel density, and (4) testing of remote sensing methods of channel head identification using field data.

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2. Study area The study area is located in the northern part of the Outer Carpathians — an area known as the Flysch Carpathians due to its geological structure. The Bieszczady Mountains are the only part of the Outer Eastern Carpathians located in Poland (Kondracki, 1989). The study area consisted of the upper elevations of the slopes of the Połonina Wetlińska Range (Fig. 1). The Połonina Wetlińska Range is formed of folded flysch sediments of the Silesian unit, which is a sedimentary rock formation characterized by variable resistance and water-bearing capacity. The formation consists of strongly fractured layers of sandstone, siltstone, and claystone, which are also irregularly fragmented by faults, all of which create a discernible system of fractures. The study area is formed of thick layers of Otryt sandstone divided by thin layers of shale and thin and medium thick layers of sandstone (Haczewski et al., 2007). Otryt sandstone is characterized by a low effective porosity (1% to 8%) and a dense network of joints and fractures, the number of which decreases significantly at depths of 40 to 60 m. The water-bearing capacity of thin sandstone layers is small, and it is essentially zero for clay–marl shale (Chowaniec et al., 1983). The relief of the study area is linked to its geological structure — the attitude of rock layers, typology and strike of faults, and rock resistance (Starkel, 1969). The main ridge of the Połonina Wetlińska Range, with the highest point at 1255 m a.s.l., runs ESE-WNW and is formed of thick layers of resistant sandstone (Haczewski et al., 2007). Relative relief in the mountain range is up to 780 m. Large and deep landslides are typical of this area. They are also the headwater areas of a dense network of deep-incised V-shaped valleys. Stream flow in headwater sections can be perennial, intermittent or ephemeral supported by spring thaws or following larger rainfall events. The density of the stream network in the headwater catchments in the Polish Carpathians is high and according to different researchers reaches 1.97–5.97 km km − 2 (Soja, 2002). Relatively moderate stream density exists in headwater catchments in the Bieszczady Mountains (1.84–3.45 km km− 2 ; Siwek et al., 2009; Mocior et al., 2011). First-order streams (Horton–Strahler classification) constitute 76% of all streams in the Bieszczady study area (Siwek et al., 2009). Streams draining the Połonina Wetlińska Range are characterized by a simple regime, with high discharge during spring thaws, and are recharged by rainfall, groundwater, and snowmelt (Dynowska, 1971). The average annual precipitation may exceed 1300 mm at the highest elevations. Precipitation amounts are highest during the summer (Nowosad, 1995). Snow cover lasts an average of 150 days per year at the top of the ridge (Michna and Paczos, 1972). Flows during the year indicate that spring meltwater is the main water source of the Bieszczady rivers during major floods. Floods in the summer caused only by rainfall are short-term and much smaller than spring floods (Plenzler et al., 2011). Streams begin via several outflows found within one larger seepage spring area or several smaller seepage spring areas located close to one another. The density of springs across the Połonina Wetlińska Range is 38 springs per square kilometer and their discharge varies. The discharge of most springs is b 0.1 dm3 s−1. Fewer than 10% of all springs found across the Połonina Wetlińska Range have a discharge of more than 1 dm3 s−1 (Lasek et al., 2012). Two vegetation zones were identified in the study area: (1) forest zone featuring beech, beech-fir and beech-sycamore forests up to 1150 m a.s.l. and (2) broad meadow zone featuring alpine and subalpine vegetation found above the upper timberline (Winnicki and Zemanek, 2009). The local rural population was resettled out of the area between 1944 and 1947. This has resulted in a marked decrease in human impact, with low intensity sheep herding and logging still taking place in the region. There are no settlements at higher elevations, above

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Fig. 1. Location of channel heads in the Połonina Wetlińska Range and a geological sketch of the study area (Mastella and Tokarski, 1995).

800 m a.s.l. The area is visited by tourists on foot or horseback. The study area became part of the Bieszczady National Park in 1991 due to its environmental value and pristine character. In 1992 the area became part of a UNESCO International Biosphere Reserve. In 2008 a part of the Bieszczady Mountains became part of the pan-European network of protected areas known as NATURA 2000 (Wolski, 2009).

3. Methods The research study was performed at higher elevations (above 800 m a.s.l.) along the Połonina Wetlińska Range. The study area consisted of 18.6 km2 of terrain. The study was divided into five stages: (1) channel head field mapping — conducted annually in July–September 2010–2012,

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(2) identification of channel head catchments and their characteristics, (3) comparative studies of the drainage network and channel head location based on two sets of input data — fieldwork conducted in 2010–2012 and the most current and available cartographic materials, (4) field measurement of local slope gradients in a random sample of 20% of the channel heads identified in the first stage (2010–2012) — conducted in September 2013, and (5) statistical analysis of channel head locations and catchment characteristics. Channel heads were mapped in the field (stage 1) by walking uphill along the stream beginning at the confluence with the valley mainstem to find the channel head and mark it using GPS with a resolution of 5 m. Streams reaching elevations over 800 m a.s.l. were first identified using a topographic map, then verified in the field. Only abrupt channel heads were mapped in the field (Henkle et al., 2011; Fig. 2). Discharge type (perennial, intermittent, ephemeral) was specified for the section downstream of each identified channel head based on specific physical indicators. The discharge type specification was based on observation during dry conditions (confirmed with the Hydrological Forecast Office's current water data website — http://monitor.pogodynka.pl/ hydro/start). However, at some field sites, this method does not give explicit results and there is potential of misidentification between perennial and intermittent flow. For streams identified as with perennial flow, spring discharge was measured via the bucket method. For these springs where flow is too limited to measure via the bucket method, a value of b0.1 dm3 s−1 was determined. The valley type was also recorded for each channel head: V-shaped valley, gully or rock veneer characterized by a high width-to-depth ratio with coarse surface material. Catchments contributing to each channel head were identified in Stage 2 using a topographic map at a scale of 1:10,000 (Mapa topograficzna 1:10 000, 1983). A triangulated irregular network (TIN) model was derived using data from the Regional Documentation Center for Geodesy and Cartography. The TIN was created using the Land Parcel Identification System (LPIS) program based on photogrammetric aerial photographs at a scale of 1:13,000 in the southeastern part of Poland. A grid-based DEM was generated with a resolution of 10 m on the basis of TIN using ArcGIS 10.1 software. The DEM was used to identify physiographic parameters for each catchment including surface area, exposure, minimum height, maximum height, relative relief, mean slope gradient, axis gradient, length, perimeter, width, index of elongation, index of circularity, topographic wetness index, Melton ruggedness number, energy index, and degree of forestation. The above parameters are widely used to characterize catchments both large and small (Marchi and Dalla Fontana, 2005; McNamara et al., 2006; Mesa, 2006; Ozdemir and Bird, 2009). A comparative analysis of field data versus the drainage network produced using a topographic map and DEM was performed (stage 3). The following elements of the drainage network were identified on the topographic map at a scale of 1:10,000: (1) stream network marked

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using blue lines, (2) dry stream sections with discernible banks marked using a special signature, and (3) potential locations of periodically drained channels marked based on the curvature of contour lines. ArcGIS 10.1 software was used to vectorize the drainage network at a scale of 1:5000. Once the drainage network had been identified using the procedure described earlier, channel heads were identified wherever appropriate. Roads and tourist trails were not included in the analysis as part of the drainage network. Drainage networks based on the DEM were generated using SAGA GIS 2.0.8 and ArcGIS 10.1 software with the ArcHydro Tools and TauDEM extensions. Pits in the digital elevation model were filled. Next, an accumulation flow network was generated based on four algorithms: (1) Eight Direction Pour Point Model (D8), (2) Random Eight-Node (Rho8), (3) Multiple Flow Direction (MFD), and (4) D Infinity (DI; Tarboton, 1997; Urbański, 2008). The next step in the drainage network identification process included the demarcation of the upslope contributing area. Using the method of repetitions for different values of the catchment area, the best results were obtained for areas of 0.5 km2 (5000 pixels). A random sample of 20% of the channel heads initially identified in the field was selected for further analysis (stage 4). Local slope gradients were measured above and below each channel head in the sample over a distance of 20 m from the channel head. The gradient of the stream channel immediately downstream of each channel head was also measured. In the case of deep seepage spring areas (more than 2 m), the inclination of the back wall was also measured. All measurements were performed using an inclinometer and measuring tape. Catchment parameters and local gradients measured in the field were normalized and standardized (stage 5). Coefficients of correlation were calculated using STATISTICA 10 software. Only correlations at the level of p ≤ 0.05 were assumed to be significant.

4. Results 4.1. Location of channel heads across the Połonina Wetlińska Range A total of 401 channel heads were field mapped on the main ridge of the Połonina Wetlińska Range at elevations between 800 and 1170 m. Most of the mapped channel heads (95%) were identified in the forest zone and only 5% above the upper timberline. Due to the topography of the mountain range, the spatial distribution of channel heads is balanced, with 51% on the northern slopes and 49% on the southern slopes. Most channel heads were found close to the axis of each valley marked on the topographic map. More than half of the mapped channel heads were located b100 m away from the thalweg (Fig. 3). The largest concentration of channel heads was observed on slopes with a gradient of 15° to 25° (Fig. 3). However more than 55% of the study area is covered by slopes with an angle between 13.5° and 24°.

Fig. 2. Examples of abrupt channel head in the Połonina Wetlińska Range.

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Fig. 3. Spatial distribution of channel heads according to slope gradients (A) and distance from the thalweg (B).

According to the topographic map (scale: 1:10,000), the study area includes 79 channel heads. By including dry stream channels and the curvature of contour lines, a total of 138 channel heads were identified on the same map, which is still only 34% out of the 401 channel heads identified in the field. The channel head density based on field mapping is 22 per square kilometer, which is five times more than the density estimated using a topographic map (Table 1). On the other hand, modeling based on a DEM provides much more outcomes than does field mapping. Furthermore, the location of channel heads identified using a DEM does not match that identified via field mapping (Fig. 4). The channel head density obtained from a DEM may be as much as 250% higher than that obtained using field data (Table 1).

4.2. Analysis of morphometric parameters of catchments contributing to channel heads Catchments contributing to channel heads mapped across the Połonina Wetlińska Range are small, ranging from 0.0003 to 0.14 km2 in area, with the mean value being 0.02 km2 (Table 2). Most (76% of cases) are forested catchments of variable shape, usually elongated, as shown by low values of both the index of circularity (0.29) and the index of elongation (0.38). The catchments are small and characterized by high slope gradients, with the mean value being 19°. Therefore many of the studied catchments (49% of cases) are characterized by high values (N1) of the Melton ruggedness number and most of them (73% of cases) are characterized by high energy index (N20; Table 2). The parameters of the studied catchments do not appear to be related in a simple way. Neither the mean slope gradient nor catchment gradient along its axis and catchment surface area appear to be related (Fig. 5). There does exists a moderate positive correlation (r2 = 0.67) between catchment area and the length of the catchment axis. A strong correlation has been noted for the energy index and catchment surface area (r2 = 0.87). Catchments with a higher axis gradient are characterized by stronger morphogenetic processes, as expressed by the Melton ruggedness number (r2 = 0.62). However, no clear relationship has

been established between the energy index and the Melton ruggedness number (Table 3).

4.3. Landform analysis in areas adjacent to channel heads The most common landform (63% of cases) found downstream of a channel head is a gully — a small and narrow but relatively deep incised and channelized valley (Goudie, 2006, p. 503). A V-shaped valley (bigger and deeper than gully and usually cut into bedrock) occurs less often (24% of cases). The least common landform is the rock veneer (13% of cases). Not every channel head is the starting point of a stream. Perennial discharge below the channel head is very common in Vshaped valleys and rock veneers. However, in the case of gullies, more than 60% are characterized by intermittent or ephemeral discharge (Fig. 6). Most springs situated in channel heads produce little discharge (b1 dm3 s− 1), while only a few produce a larger quantity of water per second (2–18 dm3). This is consistent with previous observations that most springs in the Polish Flysch Carpathians yield b 1 dm3 s− 1 (Lasek et al., 2012). The second stage of field research focused on 81 channel heads selected randomly from the study group of 401 field mapped channel heads. Various landforms were analyzed close to each channel head. This included the slope above and below the channel head and valley type. Almost 70% of the selected channels begin with large and deep seepage spring areas (up to 7.8 m deep). Gradients of the seepage area back wall were measured for landforms more than 2 m deep. Local

Table 1 Number and density of channel heads derived using different methods. Cartographic data

Derivation method

Topographic map

Blue lines + channel signatures Blue lines + channel signatures + contour lines deflection D Infinity Multiple flow direction Eight direction pour point model Random eight-node Field mapping

DEM

–

Number of channel head

Channel head density [1 km−2]

79 138

4 7

538 593 693 1005 401

29 32 37 54 22

Fig. 4. Fragment of drainage network generated by four different methods based on DEM.

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Table 2 Physiographic parameters of catchment contributing to channel heads. Parameter of catchment

Symbol

Formula

Mean

Median

Min

Max

St. Dev.

Area [km2] Mean slope gradient [°] Perimeter [m] Length [m] Axis gradient [m/m] Min height [m] Max height [m] Relative relief [m] Width [m] Index of circularity [−] Index of elongation [−] Topographic wetness index [−] Melton ruggedness number [−] Energy index [−] Degree of forestation [%]

A Sp_m P L Sp_a Hmin Hmax ΔH W Ic Ie TWI MRN EI F

– – – – =ΔH/L – – =Hmax–Hmin – =4π(A/P2) =2/L√(A/π) =ln(A + 1/tanSp_m) =ΔH/√A =A0,5Sp_a =100 forest area/A

0.02 18.92 791.98 365.36 0.32 983.63 1101.62 117.98 38.48 0.29 0.38 0.01 1.10 35.60 72.92

0.01 18.92 713.72 328.89 0.32 970.00 1096.00 107.00 29.70 0.26 0.35 0.00 0.99 32.03 88.69

0.0003 3.10 118.76 55.00 0.08 879.00 927.00 6.00 4.23 0.06 0.16 0.0003 0.11 1.91 0.00

0.14 31.04 2490.72 1161.09 0.60 1170.00 1254.00 369.00 198.24 0.80 1.61 0.03 2.59 130.16 100.00

0.02 4.71 441.05 215.71 0.08 69.27 86.71 74.31 30.02 0.15 0.16 0.00 0.50 21.31 32.95

angles vary widely from several degrees to more than 30°. The largest back wall gradient was 41° (Table 4). A weak but statistically significant (p ≤ 0.05) relationship was observed between angles close to channel heads and catchment areas contributing to channel heads. Slope gradients below channel heads and gradients of the seepage area back wall, as well as channel gradients immediately below channel heads, decrease with increasing catchment area (Fig. 7). The largest gradients can be found in seepage spring areas. Slopes above channel heads are characterized by larger gradients than slopes and channels below channel heads (Table 4). As catchment areas increase, channel and slope gradients below channel heads decrease. However, slope gradients below channel heads are smaller than channel gradients in the case of a contributing area larger than 0.02 km2. The relationship between local gradients close to channel heads measured in the field and contributing area is negative, while the relationship between gradients generated using a DEM and contributing area is positive (Fig. 8). 5. Discussion As our results have shown, the surface areas of catchments across the Połonina Wetlińska Range, ranging from 0.0003 to 0.14 km2 (mean 0.02 km2, median 0.01 km2) are much smaller than those cited in the literature: 0.01–0.6 km2 (mean 0.1 km2; Henkle et al., 2011), 0.1–0.3 km2 (Benda and Dunne, 1997), 1 km2 (Montgomery and Foufoula-Georgiou, 1993; Gomi et al., 2002; Stock and Dietrich, 2003; May and Gresswell, 2004). On the other hand, upslope contributing areas found in semi-arid to humid zones, where precipitation is characterized by seasonality, are similar to those found in the Polish Flysch Carpathians: 0.0001–0.1 km2 (Montgomery and Dietrich, 1988, 1989), 0.0001–0.07 km2 (McNamara et al., 2006), 0.0001–0.1 km2 (Tarolli and Fontana, 2009), 0.001–0.16 km2 (Jaeger et al., 2007), and 0.002 km2

(Ijjasz-Vasquez and Bras, 1995; Hancock and Evans, 2006). However, not all studies confirm this relationship (May and Gresswell, 2004), which shows that climate is not the only factor. Analogous values of contributing areas occur in the case of the Akaishi Mountains (Japan), located in a humid climate and built of flysch. Surface areas contributing to channel heads in the Akaishi Mountains are within the range of 0.0004–0.15 km2 (Imaizumi et al., 2010). In the case of two physiographic provinces in the United States (Appalachian Plateau, Ridge and Valley) examined by Julian et al. (2012), also located in a humid climate and built of flysch, the contributing areas are larger: 0.003–0.5 km2. In these two regions, the local slope gradient at the channel head and average slopes of contributing areas are smaller than those in the Polish Flysch Carpathians: 1–23.3° and 3.6–8.1° (Julian et al., 2012). The mean slope gradient of all the catchments contributing to channel heads in the Połonina Wetlińska Range is 19°. The mean slope gradient above channel heads, measured in the field, was 20° and the mean gradient of channels below channel heads was 18°. The above values are relatively high compared to other regions. Shreve (1974) proposed an average gradient of 11.3°. According to Church (2002), if a gradient is larger than 5.7°, then sediments may shift as a result of fluvial processes and will only be stopped by debris barriers. However, channel heads that possess small catchment areas are characterized by high slope gradients: 9–44° (McNamara et al., 2006), 10.8–45° (Montgomery and Dietrich, 1988, 1989), and 15–44° (Imaizumi et al., 2010) and a very high density resembling that in the Polish Flysch Carpathians: 21.3 km−2 (McNamara et al., 2006) and 32.4 km−2 (Montgomery and Dietrich, 1989). However, the density of channel heads may be even larger: 120–350 km−2 (Hancock and Evans, 2006). The channel head density in the Akaishi Mountains, characterized by humid climate and similar geological and topographic conditions to the Polish Flysch Carpathians, is much smaller — 8.4 km−2 (Imaizumi et al., 2010). Differences in the spatial distribution of channel heads result from the development process of entire headwater areas, and usually depend on geological structure, macro-relief, and elevation of the erosion

Fig. 5. Relationship between mean slope gradient, catchment gradient and contributing area. Broken and unbroken lines in the right panel are the confidence interval (95%) lines and fit linear respectively.

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Table 3 Correlation matrix between parameters1) of contributing areas; p ≤ 0.05. A A Sp_m Sp_a Hmin Hmax ΔH P L W Ic Ie TWI MRN EI F 1)

ns 0.10 −0.15 0.41 0.62 0.74 0.67 0.80 0.29 0.19 −0.19 −0.13 0.87 −0.26

Sp_m

Sp_a

Hmin

Hmax

ΔH

P

L

W

Ic

Ie

TWI

MRN

EI

F

ns

0.10 0.83

−0.15 ns 0.12

0.41 0.35 0.48 0.57

0.62 0.33 0.45 −0.27 0.64

0.74 ns 0.17 −0.32 0.53 0.92

0.67 ns 0.16 −0.34 0.52 0.93 0.99

0.80 −0.18 ns ns 0.22 0.24 0.38 0.25

0.29 −0.16 −0.12 0.20 −0.13 −0.34 −0.26 −0.34 0.66

0.19 −0.36 ns 0.18 −0.16 −0.36 −0.23 −0.37 0.67 0.76

−0.19 0.22 0.12 0.25 ns −0.30 −0.36 −0.36 ns 0.29 0.20

−0.13 0.61 0.62 −0.10 0.41 0.57 0.32 0.39 −0.44 −0.73 −0.68 −0.14

0.87 0.27 0.47 −0.11 0.57 0.77 0.72 0.66 0.74 0.23 0.16 −0.16 0.10

−0.26 0.12 ns −0.63 −0.65 −0.17 −0.21 −0.16 −0.28 −0.13 −0.14 ns ns −0.28

0.83 ns 0.35 0.33 ns ns −0.18 −0.16 −0.36 0.22 0.61 0.27 0.12

0.12 0.48 0.45 0.17 0.16 ns −0.12 ns 0.12 0.62 0.47 ns

0.57 −0.27 −0.32 −0.34 ns 0.20 0.18 0.25 −0.10 −0.11 −0.63

0.64 0.53 0.52 0.22 −0.13 −0.16 ns 0.41 0.57 −0.65

0.92 0.93 0.24 −0.34 −0.36 −0.30 0.57 0.77 −0.17

0.99 0.38 −0.26 −0.23 −0.36 0.32 0.72 −0.21

0.25 −0.34 −0.37 −0.36 0.39 0.66 −0.16

0.66 0.67 ns −0.44 0.74 −0.28

0.76 0.29 −0.73 0.23 −0.13

0.20 −0.68 0.16 −0.14

−0.14 −0.16 ns

0.10 ns

−0.28

Catchment parameter symbols as in Table 2.

base (Montgomery and Dietrich, 1988; Dietrich and Dunne, 1993; Goudie, 2006; Wrońska-Wałach et al., 2013). Although we did not investigate directly the impact of the climate, its changes over time or properties of the ground, a comparison of the results for different regions with a similar climate and geological conditions shows that the studied contributing areas are comparable in size. Furthermore, a topographic feature such as slope gradient can significantly affect water movement on a slope, and thus the size of the contributing area. The research results support theories that in regions with a wetter climate and higher slope gradients, contributing areas are smaller (Montgomery and Dietrich, 1988; Henkle et al., 2011). Channel head research in regions with different geology, climate conditions, and vegetation has shown a close inverse relationship between catchment surface area and local slope gradient (A/S; Montgomery and Dietrich, 1988, 1989, 1992). This relationship was used to estimate a channel head threshold value: 2

25 mbðA=bÞS b200 m; where A/b is the catchment area per unit contour length and S is the local gradient above a channel head (Montgomery and Dietrich, 1992). It must be noted that this relationship holds true primarily in areas with large terrain gradients and significant landslide activity (Montgomery and Dietrich, 1988, 1989, 1992). Such conditions characterize the area of our studies as well. In spite of this, a relationship between the contributing area and mean slope gradient or the gradient of the catchment axis cannot be

established (Fig. 5). Local gradients measured close to channel heads decrease slightly with increasing contributing area (Fig. 7). However, contributing area data and local gradient data pertinent to channel heads in the Połonina Wetlińska Range can be used to determine a threshold for channeled and unchanneled slopes in accordance with the findings of Montgomery and Dietrich (1992); (Fig. 9). The resulting relationship resembles that established for study areas in Oregon and California in the United States (Montgomery and Dietrich, 1992), which suggests that the catchment surface area of a channel head will decrease with increasing local gradient. The shape of the curve for the Połonina Wetlińska Range is different and may result from very high values of the Energy Index and Melton ruggedness number (Table 2), which are higher than threshold values for different mountainous areas: EI 14–87 (McNamara et al., 2006) and MNR 0.3–0.75 (Marchi and Dalla Fontana, 2005). Relationships between the catchment morphometric parameters of channel heads may or may not exist depending on the geographic region, and their characteristics will tend to vary (Wrońska-Wałach et al., 2013). In the case of the Połonina Wetlińska Range, the correlation between contributing area and maximum length of catchments contributing to channel heads is always positive, as predicted by Montgomery and Dietrich (1992). On the other hand, an inverse relationship between maximum catchment length and catchment gradient was not observed in the Połonina Wetlińska Range. This type of inverse relationship had been observed by Dietrich et al. (1986) in geographic regions where channel head formation is closely linked to landslide processes. The absence of such a relationship may be explained again by differences in climate conditions including different mean annual precipitation for the study areas of interest (Montgomery and Dietrich, 1989). Further analysis concerning this issue was not applied here, as the main purpose was to analyze topographic factors. As such, the influence of climate and its changes on channel head location is a recommended field for future research. Channel head formation may be strongly affected by geological structure in regions with variable geology (McNamara et al., 2006; Jaeger et al., 2007). A close relationship between local slope gradient and catchment surface area only exists in regions characterized by

Table 4 Local slopes [°] close to channel heads.

Fig. 6. Types of valley below a channel head and runoff frequency.

Landform

N

Min

Max

Mean

Median

St. dev.

Back wall⁎ Slope above Slope below Channel

54 81 81 81

8 6 6 5

41 38 35 33

27.4 19.7 18.3 18.0

26 18 18 17

7.5 6.7 6.6 5.3

⁎ Back wall of seepage spring area.

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Fig. 7. Relationship between slope gradient above channel head (S_Ab), slope gradient below channel head (S_Be), gradient of seepage area back wall (S_W), channel gradient (S_Ch) and the contributing area; p ≤ 0.05.

poorly permeable parent material and relatively non-complex geology (Julian et al., 2012). In light of the rather complex geology and resulting non-homogeneity of the parent material in terms of permeability (Chowaniec et al., 1983), it appears logical that no significant relationship between the parameters of catchments contributing to channel heads has been established for the Połonina Wetlińska Range. Land use may be another factor explaining the lack of a significant area-to-slope relationship. The study area has experienced a number of changes in land use over the centuries including logging and sheep herding. Dirt roads have been used in the Połonina Wetlińska Range since the 15th century. Whereas some became deeper and V-shaped due to morphogenetic processes, most ceased to exist over time (Kukulak, 2004; Wolski, 2009). Intensive land use, especially on steep slopes, may have helped to erode channel heads headward in the study area (Harden, 2006; McNamara et al., 2006; Wrońska-Wałach et al., 2013). The effect of intensive land use was especially pronounced prior to the area becoming part of a national park. With the decline of human impact, channel heads may have been able to shift downslope due to colluvial material filling in the initial site (Montgomery and Dietrich, 1989). The outcome of this series of events has been an irregular pattern of channel head locations across the Połonina Wetlińska Range (Montgomery and Dietrich, 1992).

Fig. 8. Relationship between gradient of different landforms measured in the field and contributing area.

Channel head research is complex due to spatial differences in the natural environment, changes in landforms over time, and human impact. The problem of scale is a basic difficulty when attempting to identify channel head locations based on cartographic or digital data (Horton, 1945; Mark, 1983). Topographic maps sometimes exclude some first-order streams and springs (Morisawa, 1957; Montgomery and Foufoula-Georgiou, 1993; Heine et al., 2004). This problem also applies to topographic maps of the Połonina Wetlińska Range (Table 1). In addition, springs are not always a reliable indicator of channel heads, as some channels begin above springs and exist in dry form (Fig. 6; Mark, 1983; Siwek et al., 2009). Morisawa (1957) argues that the identification of channel networks using topographic maps produces different results depending on the method used — channel identification based on (1) printed stream lines and (2) the curvature of contour lines. The latter method produces results similar to those obtained in the field. However, both methods are completely dependent on map quality. For that reason, channel networks should not be identified based on stream networks found on topographic maps, especially in the case of small catchments (b 0.8 km2; Morisawa, 1957). Remote sensing methods also do not provide enough information to properly identify channel heads (Table 1; McMaster, 2002; James et al., 2007). An attempt was made to generate a channel network using a DEM by using catchment surface areas (0.02 and 0.01 km2) as input values in algorithms. The resulting network was too dense and provided too much data (Fig. 4). The results obtained via remote sensing methods are largely dependent on the resolution used (Tarboton et al., 1991, 1992; Montgomery and Dietrich, 1992; Hancock and Evans, 2006; Orlandini et al., 2011). In conclusion, all channel head identification methods that do not include fieldwork may involve significant errors. In addition, these errors do not show any regularity. The DEM-derived channel head locations using different methods are both higher and lower, as well as leftward and rightward from the actual location of channel head in the field. Error estimate is therefore not feasible as a result of excessively high density and irregularity of DEM-derived channel head locations. When attempting to identify a channel head, it is important to remember that a channel head may shift spatially over time, as noted by Bull and Kirkby (2002). It may shift upslope due to intense headward erosion, which may occur when a channel head is also the starting point of a stream and the back wall of a seepage spring area shifts upslope. On the other hand, if a given valley becomes filled in with material produced by slope processes, then the corresponding channel head may migrate downslope. This may occur in the case of channel heads

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Fig. 9. Relationship between contributing area and local slopes in the Połonina Wetlińska Range: A — slope gradients above channel heads (S_Ab) and channel gradients below channel heads (S_Ch) measured in the field; B — threshold value between channeled and unchanneled slopes according to Montgomery and Dietrich (1992).

followed by ephemeral streams that can be filled in by material creeping downslope. Hence, the identification of channel heads demands not only an analysis of catchment morphometry, but also an analysis of local hydrogeomorphological processes (Wrońska-Wałach et al., 2013). 6. Conclusions The distribution of channel heads across the Połonina Wetlińska Range in the Polish Flysch Carpathians cannot be described using simple relationships. An uneven distribution of channel heads, which tend to form clusters, and a weak relationship between catchment surface area and catchment gradient in the Połonina Wetlińska Range may result from the heterogeneous lithological characteristics of flysch and the occurrence of systems of faults and fissures. The Połonina Wetlińska Range is characterized by a large number of channel heads. Most are recharged by small catchment areas — b0.14 km2. The threshold value for catchment size has been shown to be about 0.01 km2. This is also the most common catchment size for channel heads in the study area. The mean slope gradient responsible for channel head formation is 18° in the study area. Given the very small size of upslope contributing areas in the Połonina Wetlińska Range, remote sensing methods are not useful. Fieldwork is still the best method of mapping channel heads. This is probably due to the insufficient accuracy of DEMs, while an increase in resolution would not improve the accuracy of the terrain map, because of missing data for this region. Therefore, it is difficult to estimate channel initiation errors for an area with such a high density of channel heads. Moreover, we did not study the impact of subsurface factors (properties of the ground and subsurface runoff), which are likely to have a greater effect on the process of channel initiation than topographic factors, which may be indicated by the lack of a relationship between the parameters of the catchments contributing to channel heads. Acknowledgments The paper was written as part of the research program “HydroBieszczady”. The authors would like to thank all fieldwork participants. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.catena.2014.12. 033. These data include Google maps of the most important areas described in this article. References Benda, L., Dunne, T., 1997. Stochastic forcing of sediment supply to channel networks from landsliding and debris flow. Water Resour. Res. 33 (12), 2849–2863.

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