Pick Our Brain - July 2012

The Sacramento-San Joaquin Delta has been called an inverse delta - why?

The Sacramento-San Joaquin Delta is often called an “inverse delta” because it appears to be wider at the upstream end and narrower at the downstream end. The classic concept of a river delta is one where a river fans-out into a broad maze of channels and wetlands where it meets the sea. Although, in reality, river deltas come in a variety of forms shaped by their often unique geology, flow, and regional climate.

The Sacramento-San Joaquin Delta is fed by two major tributaries, the Sacramento River from the north and the San Joaquin River from the south, and several smaller rivers from the east side, the Cosumnes, the Mokelumne, and the Calaveras. These rivers first fan-out into many tidal channels and sloughs like a typical Delta but are then squeezed into a relatively narrow channel, the Carquinez Strait, by Coast Range hills before connecting to San Francisco Bay and the Pacific Ocean. This gives the appearance of a delta that is inverse, that is, wider upstream than downstream.