Bear Creek, Iowa
Considerations for Using Ecological Restoration: Sediment and Pollutant Loadings
from Agricultural Runoff
The Bear Creek watershed is located in north-central Iowa within the Des Moines Lobe, the depositional remnant of the late Wisconsin glaciation in Iowa. Bear Creek runs 21.6 miles and empties into the Skunk River. The watershed drains 17,180 acres of farmland, most of which has been subjected to tile-drainage over the past 40 years; approx imately 85 percent of the watershed is devoted to corn and soybean agriculture. Prairie vegetation originally dominated most of the watershed, except along the lower end of the creek where forests occurred. Roland, a town of 1,100 people, is the only co mmunity in the watershed, and no major recreational areas exist (IStART 1993).
The riparian zone along the creek has been severely degraded by past land use practices. This degradation has aggravated the effects of intensive agriculture on physical habitat and water quality in the creek, which have adversely affected aquatic life. Restoration and better management of the riparian zone should reduce the effects of non-point source pollution on the creek and improve stream habitat for aquatic life, as well as benefitting wildlife.
Stressors of Concern
Physical habitat in Bear Creek is adversely affected by high sediment loads, while water quality has been degraded by high concentrations of suspended solids, nutrients, and agricultural chemicals, particularly the herbicide atrazine.
The Goals for Restoration
The long-term goal of the project is to restore functioning riparian zones along the creek, which will, in turn, improve aquatic habitat, water quality and the aquatic community in the creek.
Restoration of the riparian zone will be done by helping farmers who own land along the creek develop functioning riparian zones. These riparian zones will intercept surface runoff and subsurface flow and will remove or immobilize sediment and agricultur al chemicals before they enter the creek, and the restored riparian zone will also provide wildlife habitat, food for wildlife, and high quality timber.
Two different levels of restoration research activity are taking place in the Bear Creek watershed. The Leopold Center for Sustainable Agriculture Agroecology Issue Team is using the watershed to study the condition of the riparian zone at the watershed level. The team is identifying critical riparian reaches along Bear Creek that would benefit from modified restoration and management to reduce the impact of non-point source pollution on the creek.
The Iowa State Agroforestry Research Team is working on one farm in the watershed to develop a model for restoring a multi-species riparian buffer strip that could be used along the critical reaches of Bear Creek and other waterways in Iowa and the Midwes t. The model Constructed Multi-species Riparian Buffer Strip site lies along a one kilometer reach of Bear Creek on a working farm approximately 1.5 miles north of Roland. The Constructed Multi-species Riparian Buffer Strip model will be used to help de monstrate the concept of the strip to farmers and to provide design specifications for similar buffer strips on their farms.
Constructed Multi-species Riparian Buffer Strip Design
Riparian buffer strips, such as Constructed Multi-species Riparian Buffer Strip, can be effective best management practices when designed to function similarly to natural riparian communities. Certain combinations of trees, shrubs, and grasses can functi on effectively as nutrient and sediment sinks for non-point source pollutants. Innovative designs use specially-selected, fast-growing tree species. If harvested for timber, their large root systems allow very rapid regrowth that provides continuity in water and nutrient uptake and physical stability of the soil throughout the life of the stand.
The Constructed Multi-species Riparian Buffer Strip design employs a three zone system that corresponds well to the new riparian forest buffer strip guidelines published by the Natural Resources Conservation Service. In the Natural Resources Conservation Service design, Zone 1 consists of a 4.5 meter-wide strip of undisturbed, existing or planted, forest whose major function is to maintain bank stability. Zone 2 consists of an 18 meter-wide strip of managed forest where nutrient sequestering is the majo r function and, therefore, requires vigorous growth and periodic removal of trees. Zone 3 contains a 6 meter-wide strip of grass that intercepts surface runoff and converts it to sheet flow or enhances infiltration so that runoff becomes shallow groundwa ter flow.
The Constructed Multi-species Riparian Buffer Strip consists of a 20 meter wide multi-species filter strip. Starting at the stream, five rows of trees, two rows of shrubs, and a 7 meter-wide band of switchgrass are used. In the strip design, the tree an d shrub species act as a combined Natural Resources Conservation Service zone 1 and 2. The selection of rapidly growing species, such as willow, poplar, silver maple, and green ash, ensure rapid uptake of nutrients. The frequent removal of the stems of these species on 8 to 12 year rotations removes the sequestered nutrients from the site. Because these species regenerate from stump sprouts, the root systems stay intact and above-ground biomass is rapidly regrown. As a result, soil stability is mainta ined and the surface remains intact because neither site preparation or planting has to be done for a number of rotations. The grass strip in the Constructed Multi-species Riparian Buffer Strip functions as zone 3.
Constructed Multi-species Riparian Buffer Strip Effectiveness
Iowa State Agroforestry Research Team evaluated the hydrogeological, environmental, and economic effectiveness of various configurations of the Constructed Multi-species Riparian Buffer Strip along a one kilometer stretch of Bear Creek. The Iowa State Ag roforestry Research Team is monitoring the site to test the ability of the strip system to trap sediment eroding from the cropped uplands, increase infiltration of water into the buffer strip soil, clean up contaminated water carrying chemicals that are m oving through the buffer strip, stabilize streambanks to reduce streambank erosion, increase biodiversity for improved wildlife habitat, and provide diversification of farm products.
The researchers are also monitoring various water quality parameters. Nitrate and atrazine changes in the groundwater, surface water, soil, and plants are being observed to determine the fate of chemicals moving through the buffer strip. Alkalinity, con ductivity, hardness, and pH data are collected in the tile and stream water. The researchers are also monitoring above- and below-ground growth of plants, physical and biological soil changes, and the presence of wildlife species.
The effectiveness of the Constructed Multi-species Riparian Buffer Strip appears to vary by aquifer system. The highest nitrate concentrations exist in the field tiles that drain cultivated fields. These tiles pass under the strip without plant-soil sys tem interaction. Researchers are developing a small constructed wetland that will intercept this water before it enters the stream channel and reduce nitrate levels by means of denitrification. Nitrate concentrations are also elevated in the alluvial an d shallow till groundwater systems during parts of the spring and summer. No measurable nitrate has been found in the limestone bedrock groundwater system. Creek water nitrate exceeds U.S. EPA limits during the late spring and summer after fertilizer ap plication. At the confluence of Bear Creek and Skunk River, the Bear Creek watershed can deliver as much as 3.5 MT of nitrate-nitrogen per day during high discharge events in the summer. Although the exact impact of the Constructed Multi-species Riparia n Buffer Strip on this loading has not been established, the strip does have a strong impact on the nitrate content of surface runoff.
Atrazine occurs in the alluvial and shallow till groundwater systems and in the field tiles, but does not exceed EPA limits. Atrazine concentrations are highest in the creek water, but only exceeded EPA limits during the heavy rains of June and July 1993 . Metabolites of atrazine are found in each of the water systems just mentioned.
Nitrate and atrazine concentrations in the soils above the water table of the Constructed Multi-species Riparian Buffer Strip are very low and provide a buffer zone of low chemical concentrations along the creek. It is not yet known whether the low conce ntrations of nitrate and atrazine are completely attributable to plant-soil processes working through the agrichemicals moving through the strip or because no chemicals have been applied directly to the strip. A mini-piezometer system is being installed to clarify the cause of these low concentrations.
Over the three years of the project, the infiltration rates in Constructed Multi-species Riparian Buffer Strip soils have increased as much as eight times over rates of neighboring cultivated land on the same soil. Visual evidence shows that the strip is effective at trapping sediment from upslope surface runoff, but additional research is needed to quantify this observation. The researchers concluded that willow post bioengineering techniques placed along the entire length of Bear Creek could reduce th e sediment load by as much as 50 percent.
Although many of the results of the project are preliminary, they successfully demonstrate that streamside buffer strips are an effective best management practice that will help make the agricultural landscape sustainable, and reduce non-point source inpu ts into surface waters, which in turn should produce improvements in surface water quality, aquatic habitat, and aquatic communities. The researchers concluded that similar buffer strips should be established along both sides of any perennial or intermit tent stream, as well as around lakes and ponds in and near farming activities, to reduce the adverse effects of nonpoint source pollution on surface water quality and aquatic life. The design using trees, shrubs, and native, non-bunch warm-season grasses is superior to cool-season grass buffer strips at reducing non-point source pollution. The 20 meter width is effective and also provides wildlife habitat and the potential for tangible economic benefits from biomass and fiber products. Although fast-gr owing tree species provide the most rapid control of the site, high quality hardwood species can be grown as part of the design and provide additional product options.
To date, most riparian zone research has been conducted either in existing naturally vegetated riparian zones or using cool-season grass buffer strips. The Iowa State Agroforestry Research Team project is the only one in the country that is conducting re search on a constructed multi-species buffer strip that consists of both woody plants and native grass. The preliminary results have shown that this design is superior to the all-grass buffer strip.
Issues of Cost
The Leopold Center for Sustainable Agriculture and the Iowa Department of Natural Resources provided funding to the Iowa State Agroforestry Research Team to lead this project. Grants from these two agencies of $73,166 each covered the project from 1990 t o 1993. Extensive cooperation was also received from several academic departments of Iowa State University.
Because the Constructed Multi-species Riparian Buffer Strip consists of a plant community that has to be established, all of the project objectives could not be completed within the allotted time frame of the project. However, the initial grant allowed e nough time for this establishment of the strip to take place and provided leverage for obtaining additional funding from USDA's Cooperative State Research Service Special Grants - Water Quality Program and the USDA/EPA Agriculture in Concert with the Envi ronment (ACE) program. These additional funds of $134,415 and $90,000 respectively allowed project research to continue through 1995.
| Back to Chapter 6 |