Historical hydrographic and hydrologic changes in the San Diego creek watershed, Newport Bay, California

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San Diego Creek drains about 320 km 2 in Orange County, California and presently empties into Newport Bay, an estuary about 50 km south of downtown Los Angeles. European settlement began in the late eighteenth century with cattle ranching. At that time, there were few stream channels across the plain so that runoff from the foothills tended to infiltrate through the fans and move subsurface to the lower elevations of the plain where it created artesian springs and swamps, a sump-like situation because there was no connection with the ocean at that time. With increasing areas of irrigated agriculture and, after c. 1940, increasing urbanization, storm runoff and channel erosion increased rapidly so that Newport Bay began to fill with sediment. This prompted massive remedial action in the 1970s, but there was little appreciation of the historical dimensions of the problem. Specifically, it was not understood that the stream channel system and its connection to Newport Bay were completely artificial and that the eroding channel system was supplying more than half of the sediment moving into the bay. Climatic changes were effective only within the context of human-induced landscape changes. This study demonstrates how rapidly landscape changes can occur and how short can be the collective memory of the natural history of a landscape. It also demonstrates how important historical geography can be to creating that memory. Geomorphically, the study demonstrates the role of base level and channel efficiency, and shows how a sediment sink can be transformed into a sediment source. It also demonstrates that watershed management should be informed by the historical record and landscape measurements, as well as mathematical modeling.