TEACHING WATERSHED PROCESSES IN THE PRESIDIO, AN URBAN NATIONAL PARK

Certain geologic processes are taught to both high school and university environmental science classes in the Tennesee Hollow watershed in the Presidio, San Francisco. The Presidio, an urban national park, borders the Golden Gate on the south and includes a recently rehabilitated wetland-beach-dune area on the margin of San Francisco Bay. Tennessee Hollow watershed is situated within the eastern part of the Presidio and drains an area of about 275 acres within the park. It extends into one of San Francisco's most fashionable neighborhoods. The watershed opens into San Francisco Bay via the rehabilitated wetland-dune area. Three ground water and spring-fed creeks drain the upper part of the watershed. The creek waters provide an opportunity to discuss ground water flow and the influence of faulting on it. The creeks coalesce into one stream that flows to the Bay through seasonal fresh water wetlands. Franciscan melange rocks and serpentinite underlie the watershed headwaters. Unconsolidated sands of the Upper Tertiary Colma Formation overlie the Franciscan in much of the watershed. Pleistocene dune sands are a feature near the outlet to the Bay. Students study the whole watershed to see relationships among rock units, to examine relationships between elevation and water flow, and to discuss influences of rock types on topography. Students investigate water quality of the creeks using appropriate meters and chemical tests to determine pH, dissolved oxygen, nitrate, phosphate and turbidity. Benthic macroinvertebrates are collected from stream sediments and identified to ascertain habitat environmental conditions. Stream channel banks are studied to study influence of riparian vegetation on bank stability. Seasonal changes in the watershed are recorded through studies during wet and dry seasons. Because it is an urban national park, the Presidio provides a field study site that is convenient for teaching.