Sinkhole Occurrence and Changes in Stream Morphology: An Example from the Lehigh Valley, Pennsylvania

摘要

The Bushkill Creek (Northampton County, Pennsylvania, USA) is a transitional fluvial system with characteristics attributable to both a straight channel stream and a meandering stream. Once within the Lehigh carbonate valley, the stream is an alluvial ponor (swallow hole). It is a losing stream with a significant percentage of the stream waters in a state of divergence through alluvial and glacial sediments that cover a well-developed karstic bedrock surface. The Bushkill is characterized by stretches of deep pools, riffles and periodic mid-stream bars composed of primarily pebbles and cobbles. These mid-stream bars are persistent features that can be observed in aerial photographs dating from the 1930's to the present. Aerial photographs that pre-date 1965 show a segment of the Bushkill between SR (state route) 33 and SR 2017 as having a split channel that circumvents a mid-stream bar. During the late 1960's, the two-channel stream was combined into one channel with the construction of SR 33. Throughout 2004/2005, both banks and the floodplain of the Bushkill Creek were being affected by an inordinate amount of sinkhole activity that was directly influencing the alignment of the main creek channel. It was hypothesized that the stream channel was making an effort to revert back to its pre-1960's configuration. Sinkholes that had opened along the north bank coalesced and eventually pirated water from the stream creating a small meander bend. Increased erosion due to subsequent rainfall and flooding eliminated the meander "neck" resulting in a recessed bank. This in turn encouraged a shift in the streams flow direction. The process repeated as existing sinkholes were connected with the now prograding stream creating a new stream channel. Although the sinkhole activity and subsequent stream response can be attributed to construction and mining activity as well as significant swings in precipitation amounts, the mechanism of stream migration is worth noting. This process of "sinkhole piracy" has been observed along other surface streams in Pennsylvania and what has been observed with the Bushkill Creek may serve as a time lapsed segment of stream evolution in a karstic terrain.