Bear Creek, Iowa
The Case Summary
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 Techniques
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.
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