Inference of Eco-geomorphic Processes Using Integrated Historical Data for a Rapidly Changing Mediterranean-climate River Corridor: Lower Santa Clara River, California

Environmental conservation demands reach-specific data as the basis for place-based restoration and management measures. However, areas such as the lower Santa Clara River valley have been subject to intensive modification since the at least the nineteenth century, but without regular hydrogeomorphic or ecological monitoring (e.g., flow gauging, repeat topographic surveys, vegetation mapping) until the mid-twentieth century. This gap has limited data on former river corridor form and understanding of historical eco-geomorphic processes. The result is a lack of consensus about the range of potential riparian and aquatic habitats to consider as restoration targets, their appropriate relative distribution, and the relationship between such ecological targets and physical processes on the river, past and present. To address this data gap on the lower Santa Clara River, we initiated an effort to extend the documentation of river characteristics beyond readily available early data sets (such as aerial photography from 1927) by integrating historical cartographic, textual, and visual accounts to create a heterogeneous but substantial dataset describing hydrologic, geomorphic, and riparian characteristics back to 1769 - the date of the first non-native exploration of the region. We collected over 1,000 documents, including General Land Office survey data, early maps from the county surveyor, and traveler’s accounts of the region, evaluating each source for its accuracy and reliability. These data were synthesized to evaluate the location and composition of riparian areas that persisted despite substantial flood events and impacts of rapidly changing land use, and to determine aspects of the former hydrology of the river affecting riparian pattern that are still relevant today. The data also indicate the general resilience of mainstem channel form in response to these events in contrast to the present-day eco-geomorphic system, which is far more sensitive to change. Used with caution, such historical data can help scientists and managers better contextualize contemporary processes on the river, and so provide informed decisions about the river’s future.