SEDIMENT TRANSPORT STUDIES
|Drainage tiles discharge water
into Walnut Creek.
Photos by Keith Schilling.
|Along many wooded segments, trees undercut by flowing water have toppled into the channel.|
Today, sediment is generally considered to be the major pollutant affecting Iowa's streams. The term "sediment" is used when referring to soil that is in transit across the landscape, whether it is slowly accumulating in valleys or rapidly carried by stream flow. High sediment concentrations in streams can have detrimental effects on water quality and aquatic habitat. In some watersheds, sediment is rapidly filling lakes and reservoirs.
In the Walnut Creek watershed in Jasper County, research as part of the Walnut Creek Nonpoint Source Monitoring Project is focused on evaluating sediment erosion and transport in a small Iowa basin. The Walnut Creek project began in 1995 as a monitoring program related to land restoration activities implemented by the U.S. Fish and Wildlife Service at the Neal Smith National Wildlife Refuge. Large tracts of land in the watershed are being restored from row crop to native tallgrass prairie. As of 1999, more than 2,100 acres of the 12,895-acre watershed have been planted in prairie. Because the Walnut Creek watershed was intensively farmed in the past, the restoration and monitoring project is providing a valuable opportunity to study how sediment moves in a stream channel modified by these historic practices, and how quickly water quality can be improved by land management changes.
As part of the monitoring project, stream discharge and suspended sediment concentrations are monitored daily at upstream and downstream locations along Walnut Creek. The product of stream discharge and sediment concentration is "load," a term used to express the amount of sediment passing the stream gage each day (i.e., tons/day). Studies of sediment loads in Walnut Creek show that sediment moves very rapidly downstream in the channel in response to precipitation and snowmelt. In any given year, as much as 25% of the annual sediment total will be transported out of the watershed in a single day, and 90% will be transported in any 20 non-consecutive days. Approximately 60% of the annual sediment load will move downstream in the months of May and June, and 98% of the annual sediment load will be transported between the months of February and July. Approximately 10,000 to 20,000 tons of sediment will be transported downstream each year in the Walnut Creek channel.
Recently, an intensive survey of a 7-mile segment of Walnut Creek was completed to gain insight into sediment erosion and transport processes in the watershed. The survey was conducted using a combination of Global Positioning System (GPS) equipment and computer mapping software. Channel features were located and described, including stream bank conditions, streambed material and thickness, along with debris dams, drainage tiles, cattle-crossings, and cross-section measurements. A portion of the resulting map is shown above.
Streambank erosion was observed to be particularly severe at many outside meander bends, debris dams, and cattle access areas. Severe bank erosion is typified by bare or exposed soils, obvious slope failures, and mature trees falling into the stream. Debris dams consist of the fallen trees, beaver dams, or large piles of debris that divert streamflow into bank sides and result in increased erosion. About 50% of the annual sediment load in Walnut Creek is derived from streambank erosion.
Silt, derived from local glacial deposits, is the dominant streambed material in Walnut Creek, and it tends to accumulate behind debris dams and at cattle crossings. At some locations, silty muck in the streambed is more than 1 to 2 feet deep. In channelized segments of Walnut Creek, surface water often flows on top of glacial till with little or no loose streambed material present. Increased water velocity in straightened reaches scours the streambed and prevents accumulation of streambed materials. This data has been used to estimate the total mass of sediment contained within the Walnut Creek channel. Based on annual averages of stream discharge and suspended sediment concentrations, and assuming no additional sediment inputs, it would take approximately nine years to flush the sediment stored in the channel bottom from the watershed.
|The Walnut Creek watershed is being restored to native tallgrass prairie from a landscape once dominated by row-crop agriculture. Currently, more than 2,100 acres of prairie have been planted by the U.S. Fish and Wildlife Service at the Neal Smith National Wildlife Refuge in Jasper County.|
Channel cross-section measurements made at numerous locations indicate that Walnut Creek has been incised as much as ten feet into its floodplain. This helps explain why sediment moves so rapidly downstream in the watershed. In incised channels, high streamflows are contained within the channel, instead of dispersing out onto the floodplain, and result in rapid sediment transport. Channel downcutting also helps explain the high number of debris dams in Walnut Creek. Many trees undercut by flowing water have been easily toppled due to blowdown or disease.
Sediment monitoring and stream mapping in the Walnut Creek watershed, in conjunction with land restoration at the Neal Smith National Wildlife Refuge, is providing a valuable opportunity to see if a return to prairie will reduce sediment loads over time. Can we go back to the clarity and morphology of historical Iowa streams? Data collected so far suggests that improvements will take a long time to observe. In the meantime, remember that stream conditions today echo the agricultural legacy of our past. While farming methods and land use practices have improved dramatically over the years with the advent of modern conservation practices (i.e., contour planting, terraces, grassed waterways, no-till planting, and riparian buffers), much of the land continues to bear witness to earlier farming methods. Driving around Iowa today, take a look at the streams to see if you can spot incised channels, severe bank erosion, or debris dams in the channel and stop to consider how "natural" these stream systems really are.
Photo by Neal Smith National Wildlife Refuge
Once-native plants and animals are thriving at the Neal Smith National Wildlife Refuge.
Above: In the fall of 1996, elk and bison were re-established.
Left and Below: The colorful blooms of reintroduced native prairie species already adorn the Refuge hillsides.
Photo by Roger Hill
Photo by Diana Horton
|New Jersey tea
Photo by Diana Horton