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Reducing thermal loads into waterways

Reducing Thermal Loads into Waterways

The City of Enterprise would like to educate the public to the benefits of shading the streams throughout our community. Water quality in the Lower Grande Ronde Sub-basins currently do not meet some water quality standards. The City of Enterprise is located within the Wallowa River Sub-basin, which is one of the Lower Grande Ronde River Sub-basins. One of the concerns throughout the area is high temperatures in our streams. When water quality standards are not met, the Federal Clean Water Act requires a Total Maximum Daily Load to be established, which determines how much pollution can be added to the river without exceeding water quality standards.

The (TMDL) lists potential causes of human-related stream heating in the Sub-basins of the Lower Grande Ronde River Basin. These include:

  • Near-stream vegetation disturbance or removal that reduces stream surface shading via decreased riparian vegetation height, width and/or density, thus increasing the amount of solar radiation reaching the stream surface.
  • Channel modifications and widening (increased width to depth ratios) increased the stream surface area exposed to solar radiation.
  • Reduction of summertime flows decrease the thermal assimilative capacity of streams, causing larger temperature increases in stream segments where flows are reduced.
  • Irrigation canals and ditches are often unshaded and increase the surface area of water exposed to solar radiation. Where these irrigation waters are allowed to mix with natural stream flows, stream temperatures can increase, In addition, irrigation return flows coming off fields or pastures can contribute warm waters to streams.

For most of these causes it is either not possible or not practical for the city to do anything that would have a meaningful impact on stream temperatures. For example, the city cannot augment streamflows by reducing withdrawals from the river since the city does not use water from the river, and has no authority to regulate diversions from the river or shallow aquifer. Similarly, it is not practical for the city to change the channel alignment within the city boundaries. Fortunately there is already a significant amount of shade-producing vegetation along much of Prairie Creek and the Wallowa River within the city limits, and an existing city ordinance (Section 4.180 of Article 4 of Appendix 1) establishes a riparian corridor that provides some protection of the streamside vegetation. Section 8.110 of Article 4 of Appendix 1 places significant restrictions on modifications to the Prairie Creek Floodways. The city would be glad to provide information and resources that would help property owners who are interested in improving the shade provided to the streams adjacent to their property by calling the Public Works Department at 541-426-3093.