HISTORICAL GEOMORPHOLOGY OF THE SANTA CLARA BASIN, CALIFORNIA

Many streams in the Santa Clara basin are highly modified where they traverse the urbanized flatlands and flow to the San Francisco Bay. An understanding of the historical stream patterns and geomorphic function of these streams can assist watershed managers and hydrologists in understanding and effectively mitigating persistent problems of erosion, sedimentation, and flooding, and in incorporating appropriate restoration elements into project design. We reconstructed the fluvial system along the mountain range-front and valley floor as it probably appeared prior to agricultural and urban development. Our reconstruction is based on interpretation of historical aerial photography, historical maps and surveys, and data from previous historical compilations.

Our mapping indicates that the location and extent of fluvial sediment deposited on the valley floor by large streams exerts a strong control on the location, course, and planform of tributary channels. Coyote Creek has deposited large amounts of sediment in a wide band down the center of the valley, and its natural levees and distributary lobes strongly affected the paths of other streams entering from the valley sides, producing “yazoo”-type planform patterns. Additionally, many tributary streams emanating from the mountains typically distributed into multiple channels and infiltrated on the piedmont without connecting to a major stream or to the bay. This fluvial process resulted in the recharging of groundwater and building of alluvial fans.

Development has resulted in (1) the connection of the creeks to each another and to the bay via engineered channels, and (2) alterations to the natural plan and profile of many creeks in the basin (e.g. meandering reaches have been replaced by straight channels, diversions have been dug through the natural levees of Coyote Creek).

Our efforts lead to (1) a better knowledge of the natural fluvial geomorphic conditions, drainage patterns, and process-linkages between surface water, groundwater, and sedimentation in the Santa Clara basin, and (2) a map tool that can be used to highlight areas where a creek has been artificially straightened, shortened, or re-routed, or as a guide for restoration project planners as to the natural characteristics of a particular creek or channel reach.