Restoration of urban headwaters pays dividends

Because urban streams are often ecologically degraded as a result of channelisation and pollution, restoration work is often attempted to improve ecosystem structure and function.  This usually involves remodelling of the stream channel.  Knowing that there are predictable shifts in ecosystem processes between headwater streams and the main body of the river, American ecologists investigated whether the location in a river network influences the effectiveness of stream restoration efforts.  They studied six restored stream reaches in heavily urbanised watersheds in Milwaukee, Wisconsin.  The restoration work involved replacing concrete channel linings with mixed substrates and naturalistic meanders, pools and riffles in an attempt to prevent flooding and improve water quality, aesthetics and recreational values.  In each stream a restored reach was compared with a channelised concrete reach of the same length (150-300 metres).  The study reaches spanned a gradient of stream size, with discharges ranging from 19 to 196 litres/second.  The researchers found that stream condition was strongly affected by discharge and geomorphology.  Re-establishing sinuous reaches with pools and riffles reduced stream velocities by up to 13 times and increased the transient retention of water by up to 50 times.  By boosting the time that water was in contact with the stream benthos, these changes reduced the distance that polluting nutrients (nitrogen and phosphorus) travelled before they were removed from the water.  In headwaters, there was a greater contrast in water quality between restored and concrete reaches than in larger streams.  This was because headwater restoration increased stream width disproportionately, so that retention of water and nutrients was more efficient in the upper reaches.  These findings suggest that restoring headwaters can provide a relatively high return on investment by maximising the extent to which water quality is improved downstream.

Reference:  Levi, P.S. & McIntyre, P.B.  2020.  Ecosystem responses to channel restoration decline with stream size in urban river networks.  Ecological Applications 30(5), e0210.7   https://doi.org/10.1002/eap.2107

Posted in: disturbance, ecosystems, habitat diversity, habitat loss, hydrology, nutrients, pollution, restoration, urbanisation, water quality