The old Edwards dam site on the Eames Road in Benton is a prime example.  You’d swear that no alewife would make it over but they do.

Alewives are very capable swimmers.  They are not considered jumpers (i.e. salmon) however they will jump on occasion.  If there is a hydraulically plausible path over or around an obstacle, the alewives will find it.  Will all make it?  No, the more difficult the obstacle (man-made or otherwise) the fewer fish successfully make it through.  On longer reaches this can have a “compounding interest” result e.g. 25 mile stream.  Beaver dams, cascades, tree fall etc.  1,000 Alewives come to obstacle #1 90% make it past = 900. 900 alewives make it to obstacle #2 and 86% make it past = 774.  774 alewives come to obstacle three, a low head beaver dam and only 50% make it over = 337.  So from 1,000 to 337 in only three obstacles.  Each scenario is unique depending on location and other abiotic factors such as flows & temperature.  The old Edwards dam site on the Eames Road in Benton is a prime example.  You’d swear that no alewife would make it over but they do.  Just not in uninterrupted droves were it just a “plain” low grade stream course with a well-defined thalweg.  These obstacles can also effect sex ratio.  Males seem to be a little more capable than gravid females.  So, potentially fewer females might go on to spawn thereby impacting juvenile recruitment and subsequent adult recruitment four years down the pike.

Nate Gray