Mississippi River - Pool 7

The Black River discharges into Pool 7 of the Mississippi River. Within the Wisconsin DNR, the Mississippi - Lower St. Croix Team addresses river issues. Three federal agencies also conduct management and monitoring activities in the river: U.S. Fish and Wildlife Service, U.S. Geological Survey and U.S. Corps of Engineers. The Environmental Management Program (EMP) has a field station in Onalaska.

This monitoring program consists of staff from several federal and state agencies with general oversight by the Corps of Engineers. Numerous management and monitoring plans exist for the Mississippi River. More detailed discussions of management issues can be found in these plans and reports generated by the agencies which manage the river.

Zebra mussels invaded the Great Lakes via cargo ships from Europe in the 1980s. They grow to between 1/8 to two inches long and have a yellowish or brownish shell with alternating bands of yellow or brown. This freshwater clam is very prolific and can form colonies with thousands of animals on boat hulls, native mussels or any submerged hard surface. Zebra mussels are present in the Mississippi River system. Presently, they are not considered a nuisance in Lake Onalaska. Boaters who use the lake should be aware of their potential to transport this destructive species to other waterbodies without careful cleaning of their boat and trailer. Boaters who plan to be on inland waters after boating in Lake Onalaska or any other portion of the Mississippi River, should conduct the recommended cleaning procedures to reduce the likelihood of transport.

1. Drain all water from boat and trailer, including bilges, live wells, bait buckets, anchor ropes and engine compartments.
2. Inspect all surfaces of the boat and trailer that come in contact with the water.
3. Remove any zebra mussels or plant material and deposit in a waste receptacle.
4. Dry boat and trailer (including anchor ropes) in a sunny location for several days before transferring to a new water system.
5. If the boat must be transferred to an uncontaminated body of water the same day, thoroughly wash boat and trailer with hot water. A car wash facility will work.
6. Do not forget the origin of the water in the baitfish bucket. Dump and clean before using in uncontaminated waters.

Lake Onalaska

This 7,700-acre lake is bounded by Lock and Dam 7 on the south, Wisconsin on the east, the Black River delta to the north and barrier islands that define the navigation channel of the Mississippi River on the west. The lake receives flow from both the Mississippi and Black Rivers. Halfway Creek also discharges to the northeastern part of the lake. The U.S. Corps of Engineers restricted the Mississippi River inflow to the lake to help maintain the navigation channel. Lake Onalaska management issues are dealt with by the Mississippi-Lower St. Croix Team within the Wisconsin DNR and several federal agencies.

Loss of fish habitat, specifically for centrarchids (sunfish), due to excessive aquatic vegetation and sedimentation was documented around the mouth of Halfway Creek and Rosebud Island within Lake Onalaska. Large fluctuations in dissolved oxygen were caused by the photosynthetic and respiratory activity of submerged aquatic plants, with periods of near anoxic conditions (Schellhaass & Sullivan 1987b). A portion of the lake was dredged near Rosebud Island in 1989 to create more fishery habitat.

The dredge spoils from this project were used to create three islands and transported inland as fill for a highway project. Dissolved oxygen concentrations have improved following the dredging project as a result of increased flows through the dredge cut channel. However, flows were initially too high and resulted in unfavorable velocity and cold temperatures in the dredge cut during winter months. (Sullivan and others, 1993). Flow modifications at the Onalaska Spillway, a part of the navigation Pool 7 control structure, were initiated following the reconstruction of the spillway in 1994-95. Winter flows have been reduced and have resulted in improved velocity and thermal conditions for centrarchids (Sullivan).

The northeastern shoreline of Lake Onalaska contains many homes. The nearshore area of the lake was historically dredged to move logs easily from the mouth of the Black River to the city of La Crosse. Dredging has not occurred in this portion of the lake for over 20 years. This Brice Prairie channel has become more shallow over the years and surface water flows have changed due to sedimentation patterns. Excessive algae blooms occur in this channel during the summer months (Endris).

A winter survey of the Brice Prairie channel in February 1997 revealed elevated levels of nitrate plus nitrite nitrogen. This channel is influenced by groundwater inflows which result in relatively warmwater conditions in this channel during the winter months. The source of these nutrients may be originating from several sources including the Onalaska landfill, septic systems and manure and fertilizer applications on nearby agricultural fields (Sullivan). The town of Onalaska, Onalaska Lake District and major industries in the Brice Prairie area should re-examine sewering all or portions of the Brice Prairie. Lakeside residents would like to see some improvements to the water quality conditions in this channel.

One aquatic plant which is vital to the canvasback duck is Vallisneria sp., also known as wild celery. Lake Onalaska at one time contained vast quantities of this plant species and was an important migration stop for the canvasback. As the worldwide population of canvasbacks shrank drastically, the Mississippi River flyway contained the last stable population. When the decline of the wild celery populations occurred after a number of drought years (1987-89), many were concerned about the demise of the canvasbacks.

A systemwide reduction of plant species occurred at the same time. It is theorized that low flows during the drought years reduced the nitrogen available to submergent aquatic plants via lower concentrations and competition with algae for nitrogen and light. Several years of low flow with reduced available nutrients stressed plants enough to cause their decline. Subsequent normal flow years produced higher suspended sediment concentrations than expected, which reduced light penetration.

For the submergent aquatic plants to rebound and thrive, the proper amount of nutrients, light penetration, flow and other factors must exist. The aquatic plant community of Lake Onalaska and the Upper Mississippi River has shown improvements in the extent of the submergent aquatic plant community since the drought years between 1987-89. Fluctuation of plant populations occur from year to year, but three consecutive drought years resulted in more severe conditions and longer recovery time for the aquatic plants (Sullivan, 1999).

Three islands were constructed in Lake Onalaska in 1989 to reduce wave re-suspension of fine sediments, provide predator-free nesting and loafing sites for waterfowl and aid in the re-establishment of aquatic vegetation beds. Each crescent-shaped island is approximately 3.5 acres in size and provides areas of diverse lake habitat that didn't previously exist within the shallow lake.

Signs of increasing use by nesting waterfowl have been noted on the relatively predator-free islands. An increase of aquatic vegetation has been documented throughout the lake. The protected areas downstream of the islands may be experiencing an accelerated rate of re-vegetation (Janvrin). Other areas of the river are also seeing an improvement in aquatic vegetation.

Recommendations

1. The town of Onalaska, the Onalaska Lake District and major industries in the Brice Prairie area should re-examine sewering all or portions of the Brice Prairie. (type c)

Black River delta

As with most large rivers, the Black River efficiently moves sediment to its mouth, creating a delta. Concerns about potential effects to Lake Onalaska resulting from changing routes and volumes of Black River water to the lake was the reason delta flows were measured in 1991-92. Flows and water elevations were measured in the main tributary and others in the delta. This data will be used as a baseline measure to compare future data against.

The 1993 flood, which was substantial for the Black River, did not significantly change flow patterns through the delta. The elevation of the railroad and HWY 35 coupled with their associated bridges and culverts essentially act as dams during high water levels. A substantial change in distributary flows is not expected due to this flow-limiting factor (Hendrickson and Haase). Portions of the Black River delta lie within federal, state and private ownership

Trempealeau Lakes area

The Trempealeau Lakes state wildlife area lies between the village of Trempealeau and the northern distributary (Tank Creek) of the Black River. The lakes are more hydrologically linked to the Mississippi River than the Black River, but they fall within the Black River basin boundary. One boat landing allows access to Round Lake. Several cottages are located on the west shoreline. This lake is heavily fished.

Round Lake suffers from sedimentation, excessive plant growth and dissolved oxygen problems during both winter (late February to early March) and summer. First Lake is a shallow wetland lake with no boat landing. The lake is bounded by the railroad on one side and the bike trail on the other. The lake outlets to Second Lake near the Trempealeau wastewater treatment plant outfall.

Second Lake has a boat landing with a handicapped fishing pier. The reason for the lack of fishery production in this lake is unknown. Third Lake contains good overwintering habitat for centrarchids. Two boat landings and a healthy fishery reflect the popularity of the lake with fishermen.