Friday, August 24, 2012

The first riffle in 200 years

August 23, 2012 - The Mill River flows freely through the former Hopewell Mills Dam spillway site for the first time in 200 years.  Like all free-flowing rivers, the Mill River now has the power to move bed material and build habitat.  We humans frequently engineer and construct habitat features like pools and riffles.  Yet, rivers have been doing that work without our help for eons.  Within a week of fully removing the spillway of the old dam, this riffle developed.  Note the slight turbulence caused by the cobbles and small boulders -- riffles aerate the river water.  Riffles are also "kitchens" where stream insects live and fish feed.  Contractors still have a lot of work to do to excavate contaminated sediment and shape the new stream channel.  But, this naturally-formed riffle reminds us that once the construction crews leave the site, Mother Nature and the river will take it from there.

In this picture, taken the same day, construction crews install dwatering pipes through the earthen berm dam.  While the spillway of the dam is gone, thousands of yards of material still fill the old floodplain.  This artificial fill will be removed so that the Mill River can regain floodplain habitat and function.