WATER LEVEL MANAGEMENT OPPORTUNITIES FOR ECOLOGICAL BENEFIT, POOL 5 MISSISSIPPI RWER¹

Abstract

This paper examines the potential to manage Mississippi River water levels for ecological benefits. The study focuses on the Weaver Bottoms, a 4,000 acre backwater marsh in southeastern Minnesota (Pool 5) highly valued for fish and wildlife habitat. The Weaver Bottoms has suffered increasing loss of aquatic vegetation and associated habitat degradation since the 1960s, largely due to persistent high water, sedimentation, wave re‐suspension of sediments, and poor light penetration. In other reaches of the Mississippi River, water level reductions exposing backwater sediments have produced strong vegetative responses due to subaerial exposure of seeds and sediment compaction. Water level management scenarios for Pool 5 were developed using the HEC‐2 water surface profile model. Results indicate that in many years it would be possible to reduce water levels sufficiently to expose much of the Weaver Bottoms, generating a substantial vegetative response. Additional benefits could be expected since both sediment compaction and increased vegetation would reduce re‐suspension of sediments. Shifting management priorities to improve habitat would temporarily impact many river users, including both commercial and recreational boaters. Water level reductions must be coordinated with their needs.

Synthesis of Upper Mississippi River System submersed and emergent aquatic vegetation: past, present, and future

“…The dewatered area in Pool 8 included over 809 ha and in Pool 5 over 404 ha, but their zones of influence extended beyond the dewatered zone as light was able to reach areas that had been too deep in previous years. The areas influenced by the drawdowns showed an increase in shallow marsh annual and perennial vegetation of 182 and 240 ha, respectively, and an increase in SAV of 47 and 580 ha in Pools 8 and 5, respectively (Woltemade, 1997;River Resources Forum-Water Level Management Task Force, 2007). Plants dominant on the exposed substrates of Pool 8 were Sagittaria L. spp.…”

“…Drawdowns also can be an effective management tool for decreasing turbidity by stimulating macrophyte growth, which decreases current velocity causing suspended solids to settle out (Dieter, 1990;James & Barko, 1990, 1994Madsen et al, 2001). Pool-wide drawdowns were first utilized in Pools 24, 25, and 26 during the summer of 1994 when water levels were lowered 0.18-0.46 m below ordinary stage (Woltemade, 1997). In 2001 and 2002, a pool-wide summer drawdown of 0.46 m (1.5 ft) was conducted in Pool 8, and another summer drawdown of 0.46 m (1.5 ft) in Pool 5 in 2005 (Woltemade, 1997).…”

“…Pool-wide drawdowns were first utilized in Pools 24, 25, and 26 during the summer of 1994 when water levels were lowered 0.18-0.46 m below ordinary stage (Woltemade, 1997). In 2001 and 2002, a pool-wide summer drawdown of 0.46 m (1.5 ft) was conducted in Pool 8, and another summer drawdown of 0.46 m (1.5 ft) in Pool 5 in 2005 (Woltemade, 1997). The second year of the drawdown in Pool 5 in 2006 was terminated early due to low water levels throughout the system.…”

Challenges in merging fisheries research and management: the Upper Mississippi River experience

“…Some of these pools are maintained on a ''hinge point,'' whereby the downstream portion of the pool is lowered more than the upstream portion and the mid-point of the pool is kept at a relatively constant water level. This arrangement allows water managers to dewater downstream shoreline areas during spring, facilitating the growth of vegetation (Wlonski & Hill, 1995;Woltemade, 1997;. This vegetation is subsequently flooded by late summer and early fall as water levels are restored.…”

20th Century Stage Trends Along the Mississippi River

“…The focus on understanding the hydrologic impacts of the river management and the potential for restoration have been based on studies conducted along the UMR (Junk et al, 1989;IFMRC, 1994;Hey and Phillipi, 1995;Johnson et al, 1995;Sparks, 1995;Woltemade, 1997). The results along the UMR and MMR are uniform because there are consistent causal influences within and adjacent to the channel, specifically locks and dams as well as levees.…”