A Mystery Solved

Two Fridays ago we went cycling on the Mount Vernon Trail. It is a lovely ride along the Potomac River that winds its way past scenic views of Washington DC and Dyke Marsh. As we were riding along I noticed a number of mounds made of mud beside the trail.




I tried to search the internet for clues as to what it might be but didn't have much luck so I started asking for help. I would like to thank Rob, The City Birder for finding the answer for us. I won't post what it is right now, because he is challenging folks to think of an answer. If you would like to take a guess as to what it was you can visit this post.

--UPDATE--
One of Rob's readers guessed it in short order: these mounds were made by a certain species of crayfish. Who'da thunk?