Section 7
Methodology Development for the Cahokia Canal/Horseshoe Lake Watershed
DRAFT 7-9
T:\GIS\Stage3\Cahokia-Horseshoe\Stage3Draft\DRAFT\Sec 7 Cahokia Cl - Horseshoe L.doc
Sanitary District (MESD), and is left open under normal conditions. There is a gate at
the MESD-owned North Pump Station that is located where the Cahokia Canal meets
the Mississippi River levee, and it is also left open under normal conditions.
During wet weather, when the Mississippi River elevation reaches 399.44 feet, the gate
at the North Pump Station is closed. When the Cahokia Canal elevation reaches
403.7 feet, flow goes over the spillway and into the diversion canal, which is located
just east of Highway 111 and Interstate 55. Cahokia Canal thereby backflows into
Canteen Lake and Horseshoe Lake. The largest flows that enter the lake during a storm
event are from Cahokia Canal. The area drained by Cahokia Canal is approximately
48,540 acres.
In early July, the 180-acre sub-impoundment is drained to stimulate growth of natural
vegetation to attract water fowl. Water is pumped out of the sub-impoundment north of
the causeway. It takes approximately a week of 24 hour per day pumping to drain the
lake. During this time, depending on the water elevations in the sub-impoundment and
the remaining portions of the lake, pumping costs can be reduced by draining the sub-impoundment
to the north. When the water level at the northern end of the sub-impoundment
is higher than the second leg of Horseshoe Lake, the causeway valve is
opened. During this time about six plant species grow up to three feet high. Water is
pumped out of the sub-impoundment after rain events (when the Cahokia Canal is
diverted into Horseshoe Lake). By late August, pumping is stopped and rain water
gradually contributes to the lake. Around September the causeway valve is opened to
let lake water gravity flow from the north into the sub-impoundment. These pumps are
also used for maintenance throughout the year. The outlet gate at the southern end of
Canteen Lake can also be closed to bring up lake levels when necessary.
The area drained by Horseshoe Lake (not including Cahokia Canal drainage) is
17,118 acres. There are three sources of inflows into Horseshoe Lake:
Stormwater runoff from surrounding residential and agricultural areas
Granite City Steel treated point source discharge
Wet weather diversion from Cahokia Canal
Stormwater runoff from Granite City discharges into Horseshoe Lake via three
culverts. There is one culvert located at the north end of the lake, which discharges
flow from Nameokie Ditch. There are two culverts at the northeast end of the lake,
which flow from Elm Slough and Long Lake. There are nine culverts at the east end of
the lake that discharge agricultural runoff into the lake. Granite City Steel uses intake
water from the Mississippi River and is permitted to discharge 25mgd of treated water
to Horseshoe Lake. This discharge is located on the west side of the lake. Runoff from
the other areas adjacent to the lake also contributes to the lake.
7.2.3.2 Global Inputs
The modeling approach for Horseshoe Lake is based on two distinct conditions; the
portion of the year when flows from the Cahokia Canal are likely being diverted into
the lake (March – May), and the remaining portion of the year which is dominated by
