Wetland Management: Drawdowns

By: Steven Hogg

September 21, 2020

Category: Resource Management

Historically, managing wetlands for wildlife consisted of drawing down a wetland entirely every 4 to 10 years to stimulate emergent and submergent plant growth. This type of management may be important, but it is only the beginning of a proper water management plan.

Water level manipulation has been documented to be critical in increasing plant and animal diversity in a wetland. The importance of hydrological change to long-term wetland health can be compared to the importance of a prairie burn to maintaining a prairie. Water management is also gaining importance because watersheds are becoming artificially modified as land around and within Three Rivers Park District is developed.

Wetlands are also facing an ever-increasing number of invasive plants with some wetlands almost entirely consisting of non-native vegetation. Changes in weather patterns and an increase in the number of extreme storm events are presenting new issues that managers must learn to navigate.

Despite these obstacles to water management, the Park District is well-positioned to provide good wetland habitat for two reasons. 

First, managed permanent wetlands found on a landscape with temporary and seasonal wetlands have been found to be incredibly valuable. The Park District has a land base large enough to provide a diverse set of wetland types. This wetland diversity becomes more important to wildlife because of the surrounding high-quality landscape of restored prairie, big woods and oak savannah.

Secondly, we currently have a total of 44 water control structures. Of the 44 structures, 35 have the potential to allow water level management on over 450 wetland acres. 

Management Goals for Drawing Down Wetlands

Management of wetland habitat is no easy task, particularly in an urban setting. State and local permitting requirements for wetland management creates not only financial costs, but also staff time and skill necessary to meet these requirements. 

In some cases, political reasons create situations where any water level manipulation is not possible. Weather can also play a role, drastically changing the outcome of even the best management plans. The physical characteristics of each wetland are unique, requiring management to be tailored to fit each site appropriately. 

Each water manipulation strategy comes down to managing three crucial elements: frequency, intensity, and timing

Some of the management goals below can be used to draw upon how to manage each site: 

Cattails and Invasive Control

In the Park District, most of the wetlands with water control structures are considered a hemi marsh — marshes that are shallow basins with interspersed emergent plants in a 1:1 ratio to open water. These wetlands are considered by wildlife managers to be ideal for wildlife. 

Unfortunately, these types of wetlands today are not providing much for wildlife because of the invasion of hybrid cattail (Typha glauca) and non-native and native Phragmites. The thick monoculture created by these plants are dense and many wildlife species simply cannot move through them. 

close-up of the feathery wetland invasive plant phragmites (common reeds)
"Phragmites" by U.S. Fish and Wildlife Service Headquarters is licensed under CC by 2.0.

They also provide little in terms of food for brood rearing and migrating wetland birds. The homogenous emergent stand of vegetation creates little in terms of a diversity of invertebrates which birds, amphibians and reptiles rely on for food.

Cattail/Phragmites control is a hot topic today and is not entirely understood. Our own wildlife staff has had success in draining a wetland entirely and then cutting the cattail using a brush saw or a machete. The stems are cut in the fall and the wetland is re-flooded before winter, resulting in drowning the cut stems and effectively killing the plant. 

This control can only be achieved on wetlands with large enough water inputs, or on sites with watersheds that allow enough water to be collected in the basin before winter. Sites without these criteria require herbicide to kill off the plants and provide space for natives.

Fish Control

In today’s artificially manipulated watersheds, waterways have become more connected which has facilitated fish to become established almost everywhere. 

Fish compete for food resources and increase the turbidity of water. This reduces plant growth and decreases the invertebrate community which is food for wetland wildlife. 

Control measures for fish include reducing the water level to less than one foot for the winter months, which will freeze the wetland and winterkill the fish. Care needs to be taken to not conduct a drawdown close to freeze-up, which can result in freezing out non-target, over-wintering wildlife such as turtles and frogs. 

This late season drawdown encourages migrating dabbling ducks and other birds that feed in the water such as herons because it will concentrate the available food.

great blue heron in the marsh
"Great Blue Heron in the Marsh" by U.S. Fish and Wildlife Service Headquarters is licensed under CC by 2.0.

Establishing and Strengthening Emergent Annuals

Managing for annual plants is a type of management that requires a full drawdown. Care must be made to keep soils moist unless the breaking of the soil is desired for tilling purposes. 

The drawdown should take place over the first third of the growing season. Re-flooding the wetland should take place after the first two thirds of the growing season and care must be taken to change water levels slowly. 

When raising the water levels, the height of the water should never exceed one third of the height of your desired growing plants. 

While re-flooding, wildlife prefer to have the majority of the site with less than 30” of water over it. The shallow water will increase use by migrating water birds. Migration starts earlier than most people realize, with shorebirds beginning to migrate in July and blue-winged teal beginning their migration in August. 

The complete drawdown and re-flooding of the wetland should last into September. The plant response will provide native annual plant seeds during migration, and brood and nesting cover the following spring.   

To a certain extent, you can dictate a desired annual plant species response by how long mudflats are kept exposed. Early exposed mudflats will favor smartweed while exposing mudflats later in the season favors wild millet and beggar’s tick.

exposed mudflats of a wetland
Exposed mudflats during a wetland drawdown at Baker Park Reserve, Spring 2019. Photo courtesy of Steven Hogg.
landscape photo of lush green marsh of beggar's tick and smartweed
Beggar's tick and smartweed annual plant response from a wetland drawdown at Baker, late August 2019. Photo courtesy of Steven Hogg.

Establishing Emergent Perennials

This wetland management is typically done as needed, usually every three to ten years. A full drawdown is completed on the site, exposing mudflats during the first third of the growing season. Ideally the drawdown should keep wetland soils saturated to having one inch of water on them. 

Drawing down early during the first third of the growing season favors Scirpus species. 

The first two thirds of the growing season will favor warm season perennials. Warm season perennials include cattails, but because of the invasiveness of these plants, this may not be a desired management strategy. This management may require multiple season full drawdowns to achieve.

Benefitting and Strengthening Submergent Plants

A partial drawdown is required for the entire growing season. If aquatic plant conditions are extremely degraded, a full drawdown may be required to compact and oxidize wetland soils and germinate aquatic plant seeds. 

The drawdown allows the light from the sun to reach the wetland bottom and increase plant growth. If curly leaf pondweed is present, then a drawdown over the winter will kill this plant. An added benefit is that sago pondweed will benefit from an overwinter water drawdown.

With any of these management goals, care must be taken when drawing down water during the last third of the growing season because of the possibility of the wetland not filling before winter. 

If this is the case, a plan should be in place to leave the wetland de-watered over the winter months. Any drawdown where less than one foot of water will be left over the winter season with no inputs needs to be at these low water levels before September 15. 

This will avoid reptiles and amphibians from trying to overwinter and becoming trapped and succumbing to winterkill. The wetland should not be re-flooded before December.

When performing drawdowns in spring, water levels need to be lowered by early April to avoid bird nests from becoming exposed making them vulnerable to predation. Having water available around nesting sites is also critical for incubating and brooding females to feed during resting periods.
 

About the Author

profile picture of steven holding an osprey

Steven Hogg is the Wildlife Supervisor at Three Rivers Park District and has been working for the Park District for 13 years. After graduating from the University of Alberta with a degree in Environmental and Conservation Biology, he moved to Minnesota to marry his beautiful Minnesota bride. Steven has always had a passion and dedication for wildlife, even when he was young. This passion is what lead him into a career where he strives for the proper orchestration of research, management, and politics to ensure natural resources and wildlife are given a voice. In his spare time, which there is little of with his three kids, Steven likes to farm, hunt, and fish.

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