LINKING INCREASED URBANIZATION TO CHANGES IN STREAM MORPHOLOGY AND POOL-RIFFLE SPACING

An emerging interest in stream restoration underscores the importance in defining natural form and process of human-impacted channels. Truly effective and lasting restoration efforts are those that consider the links between a stream and its watershed before well-intentioned, but often misguided and expensive channel modifications are undertaken. Channel morphology expresses the sum of the various hydrologic and geomorphic processes occurring in its watershed. Channel width is commonly viewed as a key metric in this regard, but it remains a one dimensional measure that may not capture the full complexity of a wider range of morphometric responses. Geodetic surveys on stream reaches draining carbonates of the Great Valley in eastern Pennsylvania reaches provide enough detail to reconstruct sub-meter scale channel forms as well as the general channel shape, from which coarser-scale channel metrics like width can be compiled. These high-resolution surveys (averaging approximately 400 points per survey) allow for new three-dimensional analyses to be calculated such as reach volume and surface area. An important finding of the surveys is that the often cited pool-riffle spacing for alluvial channels in the eastern United States (e.g. Leopold et al. 1964) is longer than 5-7 times the average reach width. Furthermore, pools and riffles are subdued in their morphology and locally absent from many stream reaches. Either the pool and riffle spacing is relict from a former, wider channel or the spacing reflects the initial response of a channel that is presently in the widening process. In either case, we hypothesize that the anomalous pool and riffle morphology stems from rapid land use changes in the encompassing watersheds. Our watersheds all have mixed uses, but have experienced different degrees of urbanization at the expense of decreasing agricultural activity within the last 50 years. These changes in land-use have changed the hydrology of the streams, producing higher peak discharges and sediment loads. Concurrent studies into the characteristics of watershed hydrology are being conducted to investigate the link between the changing hydrographs and the observed stream morphologies.