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Lithologic and structural controls on the wetlands of Rodeo Creek in the Marin Headlands, Golden Gate National Recreation, California

Abstract

When considering a watershed system in the context of restoration, it is important to understand the fundamental processes controlling the form and function of the stream environment. Among these fundamental processes are the lithologic and structural geologic controls on hydrology, especially when restoration includes complex systems like wetlands. Rodeo Creek in the Marin Headlands portion of the Golden Gate National Recreation Area has undergone numerous anthropogenic changes in the past century, including agricultural forcing as well as military development. In order to investigate the way the underlying bedrock is affecting the creeks’ wetlands, the area was mapped for structural orientation and lithology. The bedrock was found to be generally oriented in a northwest to westerly fashion and dipping toward the southwest at angles ranging from 15 to 75 degrees from the horizontal. A bedrock geologic map was constructed using these data as well as existing survey work. Areas of known wetlands were then superimposed upon the underlying bedrock structure. Wetlands were found to exist in larger distributions over contacts between different rock types. Differential erosion is suspected of creating hollows within the bedrock where alluvium can collect and become saturated with groundwater creating wetlands. This holds relevance to stream restoration work, in that, this is a way to assess the spatial distribution of where wetlands naturally occur. This technique may provide guidance to restoration efforts by more effectively locating wetlands where the watershed “wants” them to be. Additionally, this may also be a way to assess the groundwater regime of similar watersheds.

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