During the past many years, beavers have made several different migrations up Little Sewickley Creek, looking for a place to take up residence. The beavers seem to prefer the stretch of creek above the intersection of Fern Hollow Road with Little Sewickley Creek Road. While in residence, their dams change the flow of the stream, causing the water to form ponds behind the dams, thus changing the habitats within the flooded areas. The beavers maintain their dams until their food supply runs low, and then relocate themselves to an area away from Little Sewickley Creek where there is a fresh food supply.
Some people are concerned about changes in the stream associated with the beavers’ activities. For example, what are the effects on the creek since it is no longer free-flowing? Does ponding from the dams cause a rise in the water temperature? If so, does a few degrees increase in temperature affect some fish species so that they can no longer live in a particular section of the stream? Is the flooding behind the dams detrimental to the area? Are property owners not particularly appreciative of the beavers’ handiwork?
Indeed, the stream is not free-flowing in the area where the dams are located. But the habitat change makes it possible for a greater variety of species to find homes there. More wetland habitat (with sedges and rushes) has developed and more reptiles and amphibians find the beaver ponds to be places to breed and hibernate because the water is very slow moving. Recent surveys by biologists from the Western PA Conservancy and staff of Sewickley Heights Borough Park have verified several wood turtle sightings since the beavers arrived in 2012. Bird species, such as wood ducks, which are more specific to wetlands, have also been observed in the area over the past two years.
By slowing the water moving downstream and therefore slowing flooding, dams cause sediment to drop out. After the beavers move on and their dams break down, the sediment begins to move downstream again, but more slowly than prior to dam construction.
Each year for over 40 years, Little Sewickley Creek has been stocked with trout, both native and non-native species. During the past few years, data collected during fish surveys suggest that a wild brown trout population is reproducing naturally in both the main stem of Little Sewickley Creek and in the Fern Hollow (southern) branch. The annual surveys have identified wild brown young trout living in the colder Fern Hollow branch while there have been virtually no wild or native young trout found in the warmer, northern branch coming from Audubon Road and Magee Road Extension.
The activities of both man and beavers contribute to the warmer water in the northern branch. Three permanent man-made dams are located upstream from the beaver dams. When dams are built (man-made or beaver-made), trees and shade are eliminated; also, the ponds have a greater surface area than the stream. As a result the lakes and ponds are slightly warmer that the stream itself. The northern branch is not as favorable for trout spawning as the cooler main branch or the cooler Fern Hollow Branch of Little Sewickley Creek. Even after the beavers move on and the vegetation changes again, there will still be some warming effects from the permanent dams.
An important aspect to keep in mind about any stream is that it is continually changing its channel. Those of us who live along Little Sewickley Creek have watched downed trees form temporary dams. The trees stay in place for weeks or years, and then wash downstream during a high water event. We have watched small gravel bars turn into islands and then join with the bank, sometimes changing the channel. Flood waters wash fresh rock and soil material downstream, and uproot trees. There is a dynamic equilibrium within the stream for pools, riffles, bends, straight stretches, which are all dependent on the geology of the watershed, the stream bed and banks.
Certainly the beavers contribute to the ever-changing character of the stream, and they become a part of the ecology of our watershed – – – until they move on.
PostScript 1: It’s interesting to note that we do not require the beavers to go through the permitting process when they find a suitable location in which to build their dams, and they are not arrested for trespass; but any person seeking to remove a dam constructed by the beavers would likely need a permit and definitely would need permission from property owners.
PostScript 2: As indicated above, there are data available from studies done during the past few years by Fern Hollow Nature Center in conjunction with the Western Pennsylvania Conservancy, the Allegheny County Conservation District, and several colleges, which identify the fish and macroinvertebrate species at different locations along Little Sewickley Creek. In addition, temperatures have been recorded along both major tributaries and the main stream. That data will be posted to our website in the near future, and can be used to evaluate the effect of the beaver construction projects.