Making the connection

Posted on December 9, 2010

Where roads cross streams, the flowing water is commonly channeled below the surface via culverts.  Although culverts are cheaper to install than bridges, they  are less ecologically sound crossing designs because they can dramatically limit the passage of fish and other fauna.  Reasons for reduced passage rates through culverts include increased flow velocities, shallow water depths in the culvert barrel, and flow disconnection due to the raising of culvert outlets above the stream bed.  A given stream can be channeled through a large number of culverts.  For example, a survey of theCheatRiverinWest Virginia,U.S.A.recorded one culvert for every 7.2 km of stream, and these structures led to the isolation of over 200 km (33%) of high quality brook trout spawning habitat in the river headwaters.   However, a  watershed-level analysis revealed that almost 50% of isolated spawning habitat could be reconnected by replacing only 20 of the 120 surveyed culverts by bridges.  Based on this finding, the authors argue in favour of strategic, holistic approaches  to stream restoration rather than purely opportunistic, reach-scale methods.  They also recommended that since culvert designs consistently acted as barriers to trout dispersal in streams with slopes above 3-5%, in such situations culvert designs should be avoided in road construction.  

Reference:  Poplar-Jeffers, I.O., J.T. Petty, Anderson, J.T., Kite, S.J., Strager,M.P. & Fortney, R.H.  2009.  Culvert replacement and stream habitat restoration: implications from brook trout management in an Appalachian watershed, U.S.A.  Restoration Ecology 17, 404–413.