The basis for the diversity of life forms found at Famosa Slough is the water that flows into and out of it. Without the circulation of good quality water, the plants and animals native to the area would not be here. Tidal water brings not only water rich in oxygen and nutrients but also carries fish eggs and crustacean larval forms that form a vital link in the food chain.

Tidal Flow

The primary source of water to the Slough is seawater that is carried by oceanic tidal action. As the tide rises, the water flows from the ocean into the estuary that runs parallel to Highway 8. This water is mixed with some of the San Diego River water and the mixture flows through flap valves into the Slough channel. Culvert pipes under West Point Loma Boulevard allow the water to flow into the Slough.

As the tide recedes, the water reverses its course and, mixed with watershed runoff, circulates and flows out of the Slough and toward the estuary and then to the ocean.

Because of the mechanical structure of the culverts and river channel, the high and low tides in the Slough occur about 2.5 hours later than they do where the channel reaches the ocean. Although ocean tides range about 8 feet between the highest and lowest points, only about 2 feet of tidal variation is observed in the Slough.

The circulation of tides in the Slough has the effect of providing oxygen, nutrients, plant material, tide-washed fish and crustaceans to the Slough and clearing the Slough of buildups of chemicals and algae.

Watershed

A secondary source of water to the Slough is from the local watershed. The San Diego River draws its waters from a watershed that reaches Julian in the east and is 430 square miles in extent. Famosa Slough collects urban runoff water from a 370-acre (0.58 square miles) watershed. A portion of the runoff water that flows through the San Diego River channel is forced by tidal action to flow into the Slough. While usually a small fraction relative to seawater, it can be significant following a rainstorm.

The local watershed provides a normally small but continuous flow of urban runoff into the Slough. Runoff enters the slough and the channel at 19 separate points. At two points, it flows through collection basins that allow the deposit of sediment and uptake of potential chemical pollutants. Additionally, trash is captured before washing into the Slough and being carried out into the ocean by the tide. Collectively, the basins provide passive treatment of about 130 acres or a third of the watershed runoff.

During rainstorms, the collection basins are inadequate to hold all the runoff. Spillways have been designed to allow the water to overtop the pond banks.