Wetland Management

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Well managed freshwater wetlands are highly productive and a necessary resource for migrant ducks and shorebirds.


Wetland Management



Humboldt Bay NWR is comprised primarily of wetland and dune units.  The majority of wetland units were historically salt marsh and those which were not diked (Jacoby Creek Unit, Eureka Slough Unit and marsh portions of the dune units) are salt marsh today.  The historic Salmon Creek Delta was diked around 1900 and currently exists as the Salmon Creek, Hookton Slough and White Slough Units.  Due to changes in hydrology and topography over the last 100+ years, the refuge now manages much of these lands as seasonal freshwater marsh.  Restoration/conversion of these marshes to the historic estuarine condition is taking place where possible.  This transition back to functional estuary is incremental for many reasons but likely will occur ultimately due to sea level rise, a seismic event(s), or some combination thereof.  


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Pictures: The left picture shows what the Salmon Creek delta looked like in 1870. The aerial photo to the right shows what the Salmon Creek delta looked like in 2009. If you look closely, you can see that there is very little evidence left of the historical channel. 

Freshwater and brackish marshes (FBM) are extremely valuable habitat types for a large variety of birds, and contribute to the abundance and diversity of wildlife found at the Humboldt Bay NWR today. These two habitat types help sustain a variety of waterfowl, shorebirds, passerines, and wading birds, as well as the raptors that prey upon them and other animals. In addition, otters, weasels, frogs, salamanders, and invertebrates use freshwater marsh habitat. 
These habitats vary greatly in size, but are either spring-fed or seasonally-flooded and highly productive for wildlife food. Emergent vegetation in these marshes, including cattails and bulrush, can range from mostly open water to almost 100 percent cover. These marshes are generally at least seasonally brackish due to the high salinity of underlying soils and/or salt spray. However, they differ from estuarine brackish marshes in having no current tidal influence.
Because the natural hydrology of the system has been altered over time due to dike construction, channelization of waterways, and addition of fill, the Refuge must actively manage units to “mimic” natural hydrological cycles and disturbance.  Management of these habitat types consists primarily of water level manipulation, vegetation mowing, and disking of the soil.  

Water Management

Salmon and Cattail Creeks serve as the primary water source for FBM on the Salmon Creek Unit, therefore the Refuge is limited in its ability to flood-up wetlands until fall rains begin and the creeks begin to flow.  Within the wetland units water control structures are used maintain water at levels preferred by multiple species of waterfowl, shorebirds, and wading birds.  For example, diving ducks and grebes prefer deeper water to forage for fish and mollusks, while shorebirds and dabbling ducks prefer much shallower wetlands where vegetation, seeds, and invertebrates are more abundant.  As the spring progresses and birds migrate from the area, we adjust the water control structures to draw down the wetlands.  As the water recedes, moist soil plants germinate and begin to develop.  These plants serve as food sources for a host of species as we flood the wetlands up again in the fall.  They produces seeds, provide habitat for aquatic invertebrates, and green forage for suites of species that use wetland habitats.
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Picture: Water structures like the one above are used to manipulate water levels and move water from one wetland impoundment to another. 

Mowing and Disking

Prior to flood up in the fall we conduct habitat mowing of wetland vegetation in some units to enhance the vegetative structure of the wetland.  This results in more open water habitat, less dense submergent vegetation, and disperses seed heads.  Mowing is also used in areas adjacent to wetlands to reduce grass stature and to provide short grass forage for Aleutian cackling geese, American wigeon, and American Coots.  Mowing is also used to provide more open viewing opportunities for the visitors in wetlands adjacent to public trails. 
The transition of wetland habitats for bare ground to being choked with robust emergent vegetation like bulrush, cattails, or tule can occur relatively quickly depending on soil and weather conditions, water manipulation, topography, and habitat management decisions.  While we do manage for dense wetland vegetation in some units, as species such as Virginia rail, sora, American bittern, and marsh wren prefer this, other units are managed to be more open.  Disking is used to set back this succession process and provide more open habitat when the unit is flooded up.  Usually, a heavy farm disk is pulled by a tractor to break down vegetation and turn over roots and rhizomes.  After treatment the unit often has a “bare ground” appearance that at first glance may not look like quality habitat.  However, after it is flooded up bird use is high due to the availability of newly exposed invertebrates, seeds, and open water.  The real benefit comes in the second year.  After the water is drawn down, the open ground is colonized by numerous seed producing plants that grow all summer until maturity in the fall.  When these units are flooded again all of those seeds are available for food and the plants themselves provide habitat for invertebrates that are also a valuable food source for a host of waterbird species.