Geology topics

Research about Walnut Creek

Source-linked reports with geographic coverage including Walnut Creek.

2 recordsLinked to original sources

USGS2018

Combining sediment fingerprinting with age-dating sediment using fallout radionuclides for an agricultural stream, Walnut Creek, Iowa, USA

Purpose The main purpose of this study was to demonstrate the utility of the sediment fingerprinting approach to apportion surface-derived sediment, and then age date that portion using short-lived fallout radionuclides. In systems where a large mass of mobile sediment is in channel storage, age dating provides an understanding of the transfer of sediment through the watershed and the time scales over which management actions to reduce sediment loadings may be effective. Materials and methods In the agricultural Walnut Creek watershed, Iowa, the sediment-fingerprinting approach with elemental analysis was used to apportion the sources of fine-grained sediment (croplands, prairie, unpaved roads, and channel banks). Fallout radionuclides ( 7 Be, 210 Pb ex ) were used to age the portion of suspended sediment that was derived from agricultural topsoil. Age dating was performed at two different scales: 210 Pb ex which can date sediment to ~ 85 years and 7 Be to ~ 1 year. Results and discussion Sediment fingerprinting results indicated that the majority of suspended sediment is derived from cropland (62%) with streambanks contributing 36%, and prairie, pasture, and unpaved roads each contributing ≤ 1%. The topsoil–derived portion of sediment (primarily agriculture) dated using 210 Pb ex has ages ranging from 1 to 58 years, and using 7 Be, a component of much younger sediment that yields ages ranging from 44 to 205 days. The occurrence of 7 Be indicates that some portion of the sediment is young, on the order of months, whereas the dating based on 210 Pb ex indicates that some of the surface-derived sediment has been in channel storage for decades. Published studies in Walnut Creek indicate that a large component of sediment is stored in the channel bed. Conclusions We conclude that the 210 Pb ex -based ages are a reasonable estimate for the mean age of the surface-derived fraction and that 7 Be activities are evidence that there is a smaller fraction of very young sediment in the stream. We propose a geomorphic model where agricultural soil is delivered to the channel and conveyed to the watershed outlet at three time scales: a geologic-millennial time scale, decades, and a young time scale (< 1 year).

Iowa↗

USGS2007

Application of GPS and GIS to map channel features in Walnut Creek, Iowa

A 12-km reach of Walnut Creek was mapped at the Neal Smith National Wildlife Refuge in Jasper County, Iowa to identify and prioritize areas of the stream channel in need of further investigation or restoration. Channel features, including streambank conditions, bottom sediment materials and thickness, channel cross-sections, debris dams, tile lines, tributary creeks, and cattle access points, were located to one-meter accuracy with global positioning system (GPS) equipment and described while traversing the stream. The GPS data were exported into a Geographic Information System (GIS) format, and field descriptions were added to create a series of coverages. Channel features were coupled with existing land cover data for analysis. Left and right streambank erosion rates varied from slight in many areas to severe at outside meander bends, debris dams or cattle access points. Erosion estimates from this study suggest that stream banks contribute about 50 percent of the annual suspended sediment load in the channel. Substrate materials varied from bare or thinly mantled pre-Illinoian till to thick silty muck (> 0.3 m) behind some debris dams and cattle access points. Occurrences of sand and gravel areas were generally restricted to cattle access areas and bridge crossings. A total of 81 debris dams were identified in the stream channel, ranging from fallen trees and beaver dams to several large debris dams. Numerous tile lines (52 total) and tributary creeks (45 total) were mapped as contributing flow to the main channel. Cross-sections measured at 34 locations indicated Walnut Creek averages 10.64 m wide and 2.77 m deep, with the width and depth increasing downstream. Channelization and tile discharge in row crop land use areas have contributed to increased bed degradation and channel widening throughout the watershed. The results of this study indicate the effectiveness of a one-time detailed mapping program to characterize stream system variability and identify spatial relationships among many stream characteristics. Stream survey data are being used to model watershed conditions, identify water sampling points and evaluate and select appropriate channel rehabilitation measures.

Iowa↗