<

Detailed GPS Survey of Walnut Creek: Channel Characteristics and Spatial Relationships Keith E. Schilling and Calvin F. Wolter   Abstract A detailed survey of a seven-mile reach of Walnut Creek was conducted at the Neil Smith National Wildlife Refuge in Jasper County, Iowa. Walnut Creek is an entrenched (10-12 feet) third-order stream flowing through pre-Illinoian till and Holocene alluvium in the Southern Iowa Drift Plain landform region. Channel features, including bank conditions, bottom sediment materials and thickness, channel transects, debris dams, tile lines, tributary creeks, and cattle access points were described and located with global positioning system (GPS) equipment. The GPS data was exported into a Geographic Information System (GIS) format and field descriptions were added to create a series of coverages. Bank conditions varied from slightly eroded in straightened, stable segments of the channel, to severely eroded on outside meander bends and near debris dams where streamflow is diverted into the bank sides. Detailed erosion estimates from this study suggest that stream banks contribute more than 3,600 lbs annually, or 30% of the annual suspended sediment load in the channel.  Bottom sediment consisted of bare or thinly mantled pre-Illinoian till in scoured, channelized segments, and thick silty muck (>1-2 feet thick) behind some debris dams. A sandy bottom was only observed downstream of a pasture area where the channel bottom was particularly disturbed by cattle crossings. Approximately 75 debris dams were identified in the stream channel, ranging from fallen trees and beaver dams to several large debris jams. Large debris dams at some locations consisted of dozens of fallen trees blocking the channel and constricting stream flow. Debris dams were more prevalent in forested areas dominated by weak scrub trees (elm, silver maple) than in areas typified by native oak savanna. Numerous tile lines (49 total) and tributary creeks and ravines (43 total) were mapped as contributing flow to the main channel. Tiles were concentrated in agricultural areas of the basin, with some flowing between 10-20 gallons per minute. Nutrient loads from tiles and tributary creeks probably contribute to water-quality degradation observed in the main channel. Results from the bank erosion and streambed assessments are being incorporated into a GIS-based sediment-erosion model for the watershed. Other information obtained during the stream survey will be coupled with existing land cover, water quality, flow, and sediment data for further analysis.   Figure 1.  Map of Walnut Creek and accompanying photographs.     Figure 1a. Bank erosion and channel widening in a pasture area. Figure 1b. Tiles discharging into Walnut Creek.   Figure 1c. Bank erosion near debris dam. Figure 1d. Large debris dam.   Figure 1e. Stable slump blocks in channelized reach of Walnut Creek. Figure 1f. Increased bank erosion on meander cut bank.         Figure 2. Spatial relationships among variables. Spatial relationships exist between many of the channel features mapped at Walnut Creek.  Lines that connect two boxes show statistically significant correlations   (l=0.05). System variables were quantified for each ¼ mile segment of stream by either totaling numbers of features (i.e., tiles, debris dams), or averaging running totals (streambed thickness, left and right bank erosion rates) for each segment. Stream morphology was characterized by channel width and sinuosity. Channel width was determined by averaging the profile widths (see PLATE 1) measured in ¼ mile segments. Sinuosity was calculated by dividing the actual stream length by the straight-line distance between ¼ mile segments. Statistical relationships between variables were determined by regression analysis. There is evidence for scale dependence for some of the relationships. In general, dividing stream segments into ½ mile and one-mile increments leads to fewer significant relationships between variables.     Figure 3.  Bank erosion and channel characteristics. Figure 3a.  Degree of channel sinuosity and average bank erosion rate. Figure 3b. Estimated contributions of sediment to Walnut Creek.   Degree of channel sinuosity and average bank erosion rate varied considerably in Walnut Creek yet both features followed a remarkably similar pattern (Figure 3a). Sinuosity varied from one in channelized reaches to 2.3 in a highly meandered segment. In channelized areas, stream banks were well-vegetated and the annual average bank erosion rate was less than 0.05 ft/year. In meandering segments, the stream was actively eroding cut banks and creating wider channels. The average annual average bank erosion rate in these areas was greater than 0.2 ft/year. The total annual sediment load contribution from stream bank erosion was estimated to be 3,600 tons/year (Figure 3b). Based on a GIS-based sediment erosion model under development for the watershed, this sediment total represents approximately 30% of the annual suspended sediment load measured in the channel. Contributions from tributaries and gullies comprise another 30% of the annual total, and sheet and rill erosion contributes approximately 40%.     Figure 4. Streambed thickness and debris dams. The thickness of streambed materials in Walnut Creek was significantly greater in channel segments containing more debris dams. Streambed materials, consisting of silty muck, exceeded one foot in depth behind numerous debris dams located near Stream Mile 4. Where the bed thickness was less than 0.2 feet, debris dams were less prevalent and Walnut Creek flowed on top of pre-Illinoian till. Sand was the dominant channel sediment only downstream of cattle-crossing areas where the till substrate was heavily trampled and disturbed (PLATE 1).     Figure 5.  Land cover. Land use near Walnut Creek was significantly related (l=0.05) to many channel characteristics. In areas dominated by row crop (Figure 5a), Walnut Creek has been straightened and is now narrower; stream bank erosion is minimal, and the channel bottom consists of thinly mantled till. In forested areas (Figure 5b), debris dams are more numerous and streambed sediment has accumulated in the channel. Debris dams were more prevalent in forested areas dominated by weak scrub trees (stream miles 0-3) than in areas typified by native oak savanna (stream miles 3-8).       PLATE I.  Spatial relationships of GPS and GIS coverages.                                                                                                               Click to download 32" by 44" color copy of Plate I in JPG file format (1.1 megabyte).   ACKNOWLEDGEMENTS The Walnut Creek Nonpoint Source Pollution Monitoring Project is supported, in part, by Region 7 of the U.S. Environmental Protection Agency through a 319 Nonpoint Source grant to the Iowa Department of Natural Resources.  Pauline Drobney and the remaining staff at the Neil E. Smith National Wildlife Refuge are gratefully acknowledged for their considerable support of field activities.  Access to properties owned by private individuals was greatly appreciated.   Presented as a poster at The Geological Society of America 33rd Annual North-Central Section Meeting, April 22-23, 1999, Champaign, IL.