|Simply put, water level management at the Odessa Complex copies what happened on the Mississippi River before levees cut off most of the floodplain and navigation dams created a series of permanent pools. Prior to those two developments, the Odessa Complex was part of the Mississippi River with flowing side channels, interconnected backwaters and isolated wetland basins.
The same can be said for the many levee and drainage districts now farmed on the Mississippi River floodplain. All these areas once comprised a massive corridor of wetland habitat stretching the length of the river that operated under a completely different set of conditions than exists today. To understand today's water level management on Odessa, it's important to understand how a natural river system functions and how wildlife interacts with it.
Spring floods spill over the entire floodplain since there are no levees to contain them. They wouldn't be nearly as high because of the extra area to spread out over the floodplain. Fish and wildlife then take advantage of all those shallow water areas and temporarily flooded basins to spawn and feed on newly flooded food resources. Annual and perennial plants provide a substrate and food base for countless insects and other tiny aquatic organisms (referred to as invertebrates) that are high protein foods for migratory waterfowl, especially important to spring-migrating birds maintaining energy reserves for migration and egg-laying. During early spring, the decaying plants from the previous year's growth produce an explosion of these invertebrates that feed the thousands of ducks that pour through during migration. Invertebrates are also an important food for small fish and building block of the food chain for bigger fish.
As the high waters recede to the normal lower flows of summer, newly exposed mudflats are heavily used by shorebirds. As the basins begin to dry, a variety of wetland plants germinate and grow. The timing and duration of the flooding along with soil types determine the type of plants that grow. Some are annuals like smartweeds and millets that produce a heavy seed crop, while others are perennials like cattails, bulrush, buttonbush and even trees on the drier sites. Shorelines grow a thick carpet of protective vegetation. Some isolated basins contain trapped fish that become food for wading birds, turtles, water snakes, otters and a host of other wildlife as they become shallower and eventually dry. Other basins provide a home to developing frogs, toads and salamanders.
The fall normally has a slight bump in river water levels because of slightly higher average rainfall rates, and reduced evaporation and transpiration rates. That pushes shallow water into the dense beds of plants that have developed over the summer. Those beds of plants support huge numbers of migrating rails and occasional bitterns in early fall when first flooded. As the fall progresses, migrating waterfowl concentrate on the dense plant areas and feed heavily on the abundant seed produced by the annual moist soil plants.
Some persistent perennial plants like buttonbush can survive many years of shallow water once well established, but require consecutive years of dry conditions for seeds to germinate and plants to become big enough to again withstand normal seasonal flooding. Stands eventually thin and disappear during consecutive high water years, and depend on drought cycles for re-establishment. Buttonbush stands form what's known as scrub shrub wetland habitat, which is quite rare in Iowa but historically common on Odessa and heavily used by waterfowl.
Some of the highest sites on the floodplain are occupied by diverse forest stands of pin oak, swamp white oak, bur oak, pecan, shellbark hickory, hackberry, sycamore and ash, while the wetter sites are dominated by willow, silver maple, cottonwood and river birch. Although all of these species can tolerate seasonal flooding, the former group begins to decline if the root zone remains saturated during the growing season and will eventually be out-competed by the latter group, which are not nearly as valuable to a wide array of wildlife.
That entire ecosystem changed with the construction of levees and accompanying drainage systems, and the lock and dam navigation system. The river has been impounded by the navigation dams, so the summer drying cycle has been eliminated between the levees. The floodplain outside the levees has been separated from the flood pulses, and surface water is quickly removed by effective drainage systems, so the wet cycle has been eliminated there. That leaves almost none of the seasonally flooded wetlands which are the most important type for many wetland wildlife.
The Odessa Complex is located such that managers can mimic the natural water regime and benefit the numerous native fish and wildlife species that have adapted to those conditions over the centuries. An inlet is located above lock and dam 17 where water can be let in from the deeper "pool" section of the river, and an outlet is located below the lock and dam where water can be drained to the shallower "tailwater" section. High water is usually inevitable in the spring, but the inlet and outlet allow draining water out in the summer to replicate the drying cycle and running water in during the fall to replicate the fall bump. Water is also circulated through the system during the summer and winter to replicate the side channel effect which benefits the fishery.
Water level manipulation has the following benefits:
* improved habitat for waterfowl, shorebirds, wading birds, reptiles, amphibians and fish
* establishment of productive wetland vegetative communities
* consolidation, drying and firming of sediments
* healthier and more diverse forest stands and forest wildlife populations
* reduced shoreline erosion
In recent years, spillways with elevations slightly lower than Odessa's levees have been constructed. These spillways allow controlled flooding during times of high water on the river and help to reduce sediment inputs and damages to habitat and infrastructure. Additionally, fish have access to Odessa for spawning and feeding during these high water events.
Manipulating water levels is the most vital management tool used to benefit migratory birds and native fish and wildlife species.