Caring for those useful seasonal wetlands

Seasonal, sometimes isolated wetlands like this one in a pasture in the Miami River watershed of Tillamook County perform important functions that keep watersheds healthy, wildlife abundant, and water clean. Photo by Robert Emanuel, OSU.

Wetlands comprise a significant feature of the North Coast’s landscape. These include estuaries and freshwater ecosystems that are generally subject to inundation and wet soils all year. In some cases, a wetland can be seasonal too, though the definitions can become fuzzy. Tillamook and Clatsop counties are literally filled with indications of wetlands this time of year. You’ll see waterfowl, amphibians, and even some fish that manage to make it into them.  Insects, native mammals and a variety of terrestrial species depend upon seasonally flooded wetlands.

But more importantly from a watershed perspective, wetlands provide us an invaluable service. They are great at taking some of the “excess” runoff from intense storms and storing it temporarily, allowing it to either infiltrate into subsurface flow or aquifers, or by storing it as it evaporates. Furthermore, these temporary wetlands absorb nutrients and sediment before they can pollute our rivers or bays.

Unfortunately, wetlands have been replaced or filled in much of the United States.  In fact, most of the rivers that drain into our major estuaries were flooded forests or grasslands in which most vegetation was governed by seasonal floods.  The soils in much of the low elevation portions of Tillamook Bay basin, for example, would be classified as “hydric” or influenced by saturation were they not drained during the late 19th and early 20th centuries by the hard work of pioneering farm families. In many places, you can still find the plants that prefer these soils. They are what wetland ecologists use as indicators of seasonal or permanent wetlands. An example is the small-fruited bullrush (Scirpus microcarpus) below.

Small fruited bullrush (Scirpus microcarpus) is a good indicator plant of wetland soils and conditions. Photo by Robert Emanuel, OSU.

After removal of riparian forests, the construction of dike and drainage systems have  permanently changed the soils and vegetation of former wetlands into the economically important working landscapes we recognize today.  But this doesn’t mean that the wetlands are gone. In fact, looking across pastures this time year, you can still see some seasonal wetlands where drainage is poor and water pools temporarily.  And practically speaking, most farmers would prefer to keep their livestock out of these wet soils.  In the meantime, wildlife use such as breeding amphibians or waterfowl will temporarily increase as the wet and warming spring continues.

What can you do to keep your temporary wetlands functioning as flood water storage and sinks for nutrients or sediment?

1. If your seasonal wetlands appear in active pastures, try to keep livestock use light once soils are dry enough to allow animals on to them.

2. If you are grazing in or near these seasonal wetlands, rotate your livestock on and off them depending on soil saturation. If soils are too wet–such as early in the spring–then keep animals out of them. Wet soils compact easily, leading to less infiltration during the wet season and more flooding for lands adjacent to the wetland.

3. Consider fencing some areas for exclusive wildlife and recreational use, particularly if the soils stay wet for most or all of the year.

4. Establishing or allowing woody vegetation such as willows to grow on the edges or within these more saturated areas will increase flood water retention and capture more nutrient or sediment-laden runoff.

5. Watch for weeds: several nasty invasive species prefer to live in wetlands, so watch for purple loosestrife, yellow flag iris, and common reed. Once found, eradicate them immediately by  an appropriate, aquatic registered herbicide or mechanical removal.  Keep up the eradication program until you don’t see the plants return. Remember that invasives can move quickly on muddy equipment, livestock, pets or clothing.  You can find out more about some of these invaders at the Oregon Invasive Species Hotline or visiting your local OSU Extension Service office.

6. Don’t try to drain your seasonal wetlands. These are important parts of the landscape and should be enjoyed as useful for healthy watersheds, abundant groundwater, and wildlife.


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