Non-Tidal Slough Conservation

Slough work 512x219

Julia Butler Hansen Refuge has over 200 acres of Columbia River backwaters, or sloughs. The sloughs on the mainland and Tenasillahe Island were cut off from the river when the dikes were constructed. Some of the sloughs have tide gates that allow water to pass directly to the river. Other sloughs drain through ditches that connect to sloughs with tide gates. These diked sloughs have very slow current velocities because of the constricting effect of the tide gates. Water temperatures in the summer are much higher in the sloughs than water temperatures in the Columbia River. Partly because of these differences, non-native species of plants and fish are predominant. Parrotfeather milfoil covers the water surface in many areas. Introduced species of fish such as common carp, largemouth bass, yellow perch, and bluegill are abundant. Sloughs on the un-diked islands are usually open to the river at only one end (at least at normal water levels), so that current velocities are much lower than the river channels. Vegetation is generally scarce, due to the slough’s steep sides and depth. Invertebrate organisms are plentiful. Juvenile salmonids use the sloughs to forage and gain respite from strong currents. 

The refuge seeks to maintain and protect its sloughs to achieve the following characteristics: 

  • tidal exchange (with respect to magnitude, frequency, duration, and hydraulics conducive to juvenile fish passage) without flooding CWTD habitat;
  • maximum mean daily water temperature lower than 65ºF during primary period of juvenile salmon migration;
  • native aquatic vegetation (e.g., sago pondweed) and invertebrates (e.g., chironomids and gammarid amphipods);
  • relatively low abundance of nutria (a non-native, aquatic mammal);
  • relatively low abundance of carp (a non-native fish);
  • cover of less than 30 percent invasive plants including purple loosestrife and milfoils; and
  • native riparian vegetation (woody and non-woody) on banks.

Slough conservation strategies include: 

  • Install new and maintain currently existing tide gates designed to improve tidal exchange and fish passage on sloughs that are enclosed by dikes on the Mainland and Tenasillahe Island units.
  • Monitor for the presence of invasive species such as purple loosestrife, carp, nutria, and milfoils.
  • Work with partners to monitor and control invasive species such purple loosestrife, nutria, and milfoils.
  • Plant native woody and nonwoody riparian vegetation adjacent to sloughs as a source for woody debris in sloughs.
  • Recontour Risk Creek to provide more natural meanders and channel form between State Highway 4 and Brooks Slough.
  • Study the potential for rerouting Nelson Creek back to its
  • historical channel through the southeast