Illinois Environmental Protection Agency Lake Oakland and Walnut Point TMDL Development
Final Report 19
Table 3-1. Summary of Stream Types in the combined Lake Oakland and Walnut Point Lake
Watershed.
Stream Type Stream Length (ft) Percent
Canal/Ditch 24,307 38.98
Intermittent Stream 18,193 29.17
Perennial Stream 11,311 18.14
Artificial Path 8,381 13.44
Connector 173 0.28
Total 62,364 100
3.2.2 Tile Drainage
Both the Lake Oakland and Walnut Point Lake watersheds are underlain by drain tile designed to remove
standing water from the soil surface. Subsurface drainage is designed to remove excess water from the
soil profile. The water table level is controlled through a series of drainage pipes (tile or tubing) that are
installed below the soil surface, usually just below the root zone. In Illinois, subsurface drainage pipes are
typically installed at a depth of 3 to 4 feet and at a spacing of 80 to 120 feet. The subsurface drainage
network generally outlets to an open ditch or stream.
Researchers at the University of Illinois and elsewhere have studied the impact of tile drainage on
hydrology and water quality. Some impacts are relatively well understood while others are not. Zucker
and Brown (1998) provided the following summary of the impacts (statements compare agricultural land
with subsurface drainage to that without subsurface drainage):
• The percentage of rain that falls on a site with subsurface drainage and leaves the site through the
subsurface drainage system can range up to 63 percent.
• The reduction in the total runoff that leaves the site as overland flow ranges from 29 to 65
percent.
• The reduction in the peak runoff rate ranges from 15 to 30 percent.
• Total discharge (total of runoff and subsurface drainage) is similar to flows on land without
subsurface drainage, if flows are considered over a sufficient period of time before, during, and
after the rainfall/runoff event.
• The reduction in sediment loss by water erosion from a site ranges between 16 to 65 percent. This
reduction relates to the reduction in total runoff and peak runoff rate.
• The reduction in loss of phosphorus ranges up to 45 percent, and is related to the reductions in
total runoff, peak runoff rate, and soil loss. However, in high phosphorus content soils, dissolved
phosphorus levels in tile flow can be high.
• In terms of total nutrient loss, by reducing runoff volume and peak runoff rate, the reduction in
soil-bound nutrients is 30 to 50 percent.
• In terms of total nitrogen losses (sum of all N species), there is a reduction. However, nitrate-N, a
soluble nitrogen ion, has great potential to move wherever water moves. Numerous studies
throughout the Midwest and southeast U.S., and Canada document that the presence of a
subsurface drainage system enhances the movement of nitrate-N to surface waters. Proper
management of drainage waters along with selected in-field BMPs helps reduce this potential
loss.
