Water Quality Assessment
Introduction
Understanding the physical features of the watershed is important because the physical features
have a great influence on the water quality degradation in the watershed.
Sedimentation
A lake sediment survey performed by NRCS in 1988 indicated the 5,147 acre lake had an average
depth of 6.6 feet and an average sediment thickness of 3.5 feet. The lake has lost approximately
35 percent of its storage capacity. Profiles show the inlet areas are less than three feet in
depth and dredging is necessary to maintain boating access.
The immediate problem is the ongoing need to dredge the inlets of the major tributaries where
sediment accumulations are greatest. Disposal areas for dredged materials are becoming
overloaded and potential for developing feasible, newdisposal sites are lessening because of
limited land availability, long pumping distances and environmental restrictions.
The primary source of Indian Lake sediment is a 1,000 foot corridor on the tributaries.
Sediment from steeper sources follows defined drainage ways, while flatter area sediment enters
the aquatic ecosystem via sheet flow. The delivery system (stream transport system) is efficient
throughout the watershed. All sediment is carried into the lake, whether entering the system at
headwaters or by the lake.
Eroded material originating further than 1,000 feet from the delivery system moves slowly into
the system. The lake has received approximately 1,500 tons of sediment annually from each
square mile of drainage area for an estimated total of 135,879 tons. The sediments at the
inlets of tributaries are predominantly silts while sediments in open water areas are clays
and silts.
The NRCS soil erosion calculations indicate areas within 1,000 feet of tributaries and Indian
Lake have a sediment delivery rate of about 24 percent into the lake. So, for every 100 tons
of soil erosion within 1,000 feet of a stream, 24 tons of soil will erode into Indian Lake
annually. Eventually the other 76 tons of the original 100 tons will make its way through the
tributary system and into the lake.
The 1988 Ohio EPA study, using Ohio Capabilities Analysis Program results, estimated the overall
watershed delivery rate to the lake is approximately 19 percent. This means an estimated 78,919
tons of sediment were entering Indian Lake from sheet and rill erosion annually. Gully erosion
from streambanks and other areas are not figured into this estimation. Using the Universal Soil
Loss Equation formula, this loss is equivalent to an average of 7.2 tons/acre/year across the
watershed. With the high use of no-till and conservation tillage, coupled with other BMPS,
sediment entering the lake has been reduced to 31,283 tons annually. This equates to 4.9
tons/acre/year of soil loss across the watershed. NRCS reports a majority of the watershed has
a three to five ton/acre/year soil loss tolerance.
The increase from six percent to 65 percent no-till cropping in the watershed accounts for the
sizable decrease in sediment loading to Indian Lake. See Section 10 for a tillage transect
breakdown.
Sediment delivery rates are the lowest in the Van Horn and Blackhawk
subwatersheds, followed
by the North Fork of the Great Miami. The South Fork of the Great Miami has the greatest
overall sediment contribution to Indian Lake. Sediment delivery rate is affected by several
watershed factors such as topography, shape of watershed, land use, rainfall intensity, stream
gradient and soils.
In-lake sediment surveys were performed on November 2, 1988, at the inlets
of the North and South Forks of the Great Miami River, and at the mouth of Van Horn Creek. The
volume and mass of dredged sediment at each location was determined and added to in-lake
statistics. Average annual sediment loadings were determined on a per square mile basis, see
table below.
Estimated Soil Erosion
Sheet and Rill Erosion vs. Sediment Delivery Rate
| Year | Soil Erosion
Tons/Year | Percentage
of Change |
|---|
| 1990 Sheet and Rill Erosion | 415,000 | - |
| 1990 Sediment Delivery Rate | 78,850 | - |
| 1994 Sheet and Rill Erosion | 200,000 | 48% |
| 1994 Sediment Delivery Rate | 40,918 | 51% |
The 1990 data was estimated through National Resource Inventory data and Ohio Capability
Analysis Program for sheet, rill and gully erosion. Sheet and rill erosion in 1994 were
approximated from Indian Lake tillage and crop residue data and was calculated through the
Universal Soil Loss Equation (U.S.L.E.).
Gully erosion has been reduced due to the implementation of numerous BMPs such as grassed
waterways, grade stabilization structures and critical area seedings.
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