Our field trip for Jai Cole’s Freshwater Ecosystems class visited the Left Fork of the Paint Branch, on the site of the former Maydale Nature Center. The site is part of the Upper Paint Branch Special Protection Area, and the stream was the target of recently completed restoration work. We focused on the restoration work, and also performed a classroom exercise-level habitat assessment.
The centerpiece of the restoration is this 100-meter reach. You’re looking upstream and roughly northwest, standing on a bridge that provides access to the area. Out of frame to the right is a small parking lot. The point of the project was to replace a series of notched logs that channelled the stream as it drops from a weir (the flat water just visible in the background) (which maintains water supply for a pair of ponds on the property) and flows under the bridge; the problem with the logs was they they weren’t designed to allow fish passage. (Brown Trout is a naturalized breeder in the watershed.) The project replaced the logs with a series of arcing rock structures (called “cross-veins” in the local engineering parlance), each with a gentler drop and a plunge pool downstream. The pools give fish swimming upstream enough elbow room to get up speed to jump and surmount the rock barrier. Notice how the top of each arc of stones drops a few inches at the center: that’s where we want the most water to flow. A vertical plane through a watercourse passes through the point of maximum flow and the deepest part of the channel, which is called the thalweg, and in this case we want the thalweg to remain where it is.
From elsewhere on the stream, here’s a closeup (albeit with a lot of glare) of an arced cross-vein on the right and a straight-line weir on the left. Water flow is right to left, and the weir maintains the pool downstream of the cross-vein. Again, notice that, at this level of flow, the stones of the cross-vein near each bank are high and dry, and the stones in the center have the most flow over them.