Planning Guidance: Road Impounded Wetlands


This document is provided for archival purposes only. Archived documents do not reflect current WDFW regulations or policy and may contain factual inaccuracies.


Published: October 11, 2002

Pages: 12

Author(s): Bob Barnard

This publication has been superseded by the 2013 Water Crossing Design Guidelines. 


Road impounded wetlands are the result of undersized or perched culverts in combination with impermeable road fills that create wetland conditions in the upstream impoundment. Often these same culverts block fish and wildlife passage up and down the stream course and interrupt natural channel processes.

State law requires that road owners provide fish passage at road crossings. There are basically two alternatives to address this situation. One, lower and enlarge the culvert to create passage and encourage the continuity of stream processes (e.g., sediment and debris transport). This alternative removes the control that created the wetland and causes it to return to a stream.

The other alternative is to construct hydraulic control using artificial structures that provide fish passage and maintain either all or part of the wetland. This can be expensive, not always possible, and often not in keeping with naturally sustainable stream processes.

In spite of state law requiring fish passage in streams affected by road crossings, state and federal policies also call for a no net loss of wetland. This document is intended to help biologists, landowners and designers evaluate road crossings with wetlands impounded above them so that they may intelligently and legally choose between the two alternatives discussed above. This guidance was completed in cooperation with various concerned groups, including state and federal regulatory agencies and a number of prominent forest land owners. The focus here is overall ecological health and compliance with Washington State regulations, although one must pay careful attention to other relevant laws, including the Clean Water Act sec. 404 and local critical areas ordinances.