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Seasonal Freshwater Wetland Management at the Refuge

wetlands in winter 512x365For many visitors, it is a surprise to discover how carefully and deliberately the U.S. Fish and Wildlife Service manages habitat at the Refuge.  What practices are required to create the most beneficial habitat for visiting waterfowl?  How does the Refuge ensure that this benefit is distributed to include early and late arrivals?  Staff member Jesse Barham answers these questions and more.

As local birders likely know, October through April is the best time to view large flocks of waterfowl here at the Refuge. Nisqually NWR is primarily a migration and overwintering location for these species so these months are a great time to visit the Refuge to view a diversity of birds. The Nisqually Delta is a good place to overwinter for these birds in part because its landscape position provides protection from the prevailing southerly winds during that time of year and the mild maritime climate has relatively few days of sub-freezing weather in the winter. The estuarine habitats (salt marsh and mudflat) in the delta provide very productive feeding and resting areas for these species. The close proximity of quality freshwater habitat provides diversity for a variety of birds. The most abundant waterfowl species include American wigeon, American green-winged teal, northern pintail, and mallards which can be observed at the Refuge in winter using both fresh and saltwater habitats. Historically these freshwater habitats would have been found in areas south of I-5 that have since been converted to agricultural uses.

The management and enhancement of seasonal freshwater habitats is an important part of the habitat program at Nisqually NWR. The Nisqually NWR Comprehensive Conservation Plan (CCP) described the restoration of 762 acres of historic tidal habitat here at the Refuge and the enhancement of 250 acres of freshwater habitats behind the new exterior dike. Roughly 170 acres of grasslands and seasonal freshwater wetland habitats within the dike are managed to benefit waterfowl and other migratory birds. In 2008-2010, concurrent with the estuary restoration project and assisted by Ducks Unlimited, many actions were taken to improve management capabilities in the seasonal freshwater wetlands to provide high quality habitat and food resources for migratory birds. Seasonal wetland habitats were enlarged during the 2008-2009 construction seasons by strategically borrowing dirt to build the new exterior levee. New interior levees and water control structures were also installed to improve management capabilities. This divided the seasonal freshwater wetlands into five separate units and significantly improved management capabilities and habitat value in these areas.

A variety of methods are used to provide productive habitats for over-wintering waterfowl in the seasonal freshwater units. The basic strategy involves techniques often referred to as moist–soil management. Flooding units during the fall and winter to appropriate depths for waterfowl foraging is a key component, allowing dabbling ducks easy access to food resources. Here at the Refuge we rely primarily on rainfall to flood the freshwater wetlands, with a small contribution from an artesian well and pump that we use from September to December to provide ponded water for early migrants. The Refuge closely manages water levels as various waterfowl and other waterbird species prefer a narrow range of water depths for feeding, primarily from 2-10 inches. Our goal is to maximize the amount of ponded area at these depths when waterfowl are most abundant at the Refuge, while slowly flooding more areas to appropriate depths as food resources are depleted.

Conversely, water levels are slowly reduced in the late spring and early summer, which is a technique known as a drawdown. Draining water off of seasonal wetlands allows wetland plants to germinate and grow through the summer to produce seeds for the coming season. As water levels are reduced, invertebrate food resources are concentrated in smaller areas providing a boost for spring migrants moving through the Refuge. The timing of drawdowns is critical as various plant species will germinate as the season progresses and temperatures warm, with more desirable seed producing species often requiring warmer temperatures. It is also beneficial to wait until the cottonwood and willows have spread most of their seed before exposing the mud that has been under water all winter. Drawdowns also expose other food resources such as substrates for invertebrates. Another benefit to drawdown is that keeping areas flooded longer appears to reduce the vigor and spread of reed canarygrass. This strategy reduces the amount of maintenance required in future years, by reducing this problematic species vigor and abundance. Varying the depth and timing of inundation is a great way to encourage diverse plant communities in the units and keep them healthy.

Mowing and plowing/discing are the primary tools used to halt succession and encourage vegetation communities that will provide optimal seed resources for waterfowl. Reed canarygrass and willows can begin to dominate seasonal wetlands that are not managed periodically, greatly reducing the value of these wetlands for waterfowl. Managing these habitats every few years allows annual wetland plants to germinate, producing abundant seed resources for waterfowl. Active management also reduces perennial and woody species and encourages native moist-soil plant communities that provide the greatest benefit for waterfowl and other waterbirds. Mowing the grassland habitats adjacent to the seasonal wetlands in the late summer allows easier access for waterfowl and creates better browse for geese and wigeon.

The managed seasonal wetland habitats at the Refuge provide a complementary habitat to the estuary for waterfowl and other species in the Nisqually Delta and are the current focus of much of the Refuge’s habitat management work. Hopefully you will come on out to the Refuge this winter to observe the large numbers of birds taking advantage of these improved habitats.
Page Photo Credits — seasonal freshwater wetlands, ©John Whitehead
Last Updated: Sep 13, 2012
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