Detailed GPS Survey of Walnut Creek: Channel Characteristics and Spatial RelationshipsKeith E. Schilling and Calvin F. Wolter
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.
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%.
PLATE I. Spatial relationships of GPS and GIS coverages.
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.