Water Resources Collections and Archives

Rodeo Lagoon is located ten miles north of San Francisco in the Golden Gate National Recreation Area. The 35-acre lagoon is separated from the ocean by a 500 foot wide beach for most of the year. In a normal year, Rodeo lagoon is joined to the ocean for three or four weeks during the winter, when floods in the watershed cause the lagoon to overtop its barrier beach. At this point an inlet channel forms, bringing water in and out of the lagoon with the tide until waves again build up the barrier beach. This breaching is critical to maintaining the health of the lagoon because it flushes the lagoon water out, keeps the temperature and salinity of the water at appropriate levels, and allows fish passage (Madej 1989).

The water quality in the lagoon is currently poor. This is of concern for three main reasons: several endangered species live in or around the lagoon, it is an important stop for migratory birds, and thousands of visitors come in contact with the lagoon each month. It is probable that the poor water quality in the lagoon can be partially attributed to the lagoon breaching less often than it did historically (Brown 1993). This phenomenon is not uncommon along the California coast. According to Goodwin (1996), alterations in the watershed as well as the diking off of regions of the lagoon have compromised the stability of many inlet channels along the coast of California over the last century. As a result of development in the area by the military during World War Two, both of these conditions are present at Rodeo lagoon.

According to Ranasinghe (1998), the major influence governing the stabilization of inlet channels is the streamflow through the lagoon. In order to understand the current hydrologic situation in Rodeo lagoon better, I have characterized the water balance in the lagoon during the dry season when the lagoon is isolated from the ocean. I used the volume of inflow to the lagoon from the watershed above (2, 856.7 ft2/6hrs, measured by Oerter 2003), estimated the volume lost to evapotranspiration (2,618 ft2/6hrs), and estimated the volume seeping through the beach (between 2,071.6 ft2/6hrs and 2,071,600.0 ft2/6hrs). In these late dry season conditions, where the rate of outflow from the lagoon exceeds the rate of inflow, pollutants are progressively concentrated in the lagoon as its volume decreases until the beach is breached and tidal exchange with the ocean through the inlet channel restores water quality in the lagoon. The information gathered in this project may be used to investigate further the potential for establishing a stable inlet channel.