Peirce Mill dam on Rock Creek in D.C. was an obstacle to migrating fish until a Denali fish ladder was built in the early 2000s (right side of photo).
Photgrapher: Tom Paradis, courtesy of CUE, NPSDams and fords change aquatic habitat by creating a deeper, slower water with increased sediment upstream, and a flow with decreased sediment downstream. They also prevent some species of fish from migrating upstream for spawning.