Tuesday, May 14, 2013
Low head dams scare me. Even small ones. Uncertain scour holes and unpredictable currents can make even the most benign looking low head dangerous. I approached the fish hatchery dam on the Little Lehigh with a lot of caution. Low head dams are ecologically frightening as well. They completely change the ecology of a river. Sediments build up behind the dam and smother diverse habitat creating a monotonous sand and mud flat plane. The still water warms and becomes oxygen poor and the artificially ponded water provides habitat for artificially introduced species. Things that really don’t belong in our creeks, but are at home in a pond. And so the Wildlands Conservancy (www.wildlandspa.org) and their partners have undertaken an ambitious project that will remove 9 dams on the Little Lehigh and Jordan Creeks in the next few years. This will be a monumental step in restoring these streams to their natural free flowing condition, and I want to watch the process. Underwater. I entered the stream a hundred yards downstream of the dam and crept into the current. The bottom here is angular cobbles, all covered in an algal carpet. This stream receives excess nutrients which makes excess algae grow, and it covers everything. That doesn’t seem to bother the trout that gather in a deeper eddy in the lee of a large rock. Or the darters that flit from the shallows into deeper water as I approach. Eutrophication, or the over fertilization of our rivers and streams, is a problem. But maybe habitat diversity is more important. I continue upstream and slowly enter the plunge pool of the dam. This dam isn’t more than 2 feet tall, but still I can feel the recirculating current pull me in. I drop my feet to the bottom and hook my toes on some rocks. There isn’t any obvious life here. It is loud and full of entrained air bubbles. But there aren’t any large fish like I envisioned, at least none that I can see. It’s a violent place, and the bottom drops out of view. Dams affect downstream as much as they do upstream. Hydraulics and hydrology, nutrient cycling and sediment flows are all affected by dams. I’m pretty sure the lack of abundant life here is related to the dam. Every dam site I have snorkeled except one had less than expected fish abundance and diversity. I was confident that pattern continued here as I climbed over the concrete abutment on river left and slid into the large flat water expanse. The downstream habitat was algae covered cobble; a diverse assemblage of different sized rocks with deeper areas where water scoured around some of the large rocks and shallower area where sands were deposited. This creates opportunities for a diversity of species to set up shop and live. Diverse habitat usually means diverse biology. The upstream habitat in contrast was a monotonous sand and mud flat plane. No diversity of contour, and the biology reflected it. The dam effectively formed a desert devoid of any larger life. If there aren’t places for fish to safely hunt and hide, there won’t be any fish. As I turned to float downstream I saw an Asian clam with its foot extended, feeding. The only life in this large pool that I witnessed today. I stopped a few feet upstream of the dam and got out of the river. This 2 foot tall hunk of concrete has outlived its usefulness, and significantly altered the ecology of this river. It’s time for the dam, and the desert it formed, to go.