Following construction of Hoover Dam (1935) and Glen Canyon Dam (1964), little or no water flowed to the Gulf of California. Lake Powell, the last reservoir built on the river, finally filled in 1981, and since then occassional flood flows have resumed. During years of excess precipitation in the watershed, as during the El Niño cycles of 1983-86, 1993, 1997-99, water is spilled from the dam system to the sea, revitalizing the delta habitats along the way.

   Augmenting these flood flows, the US has discharged appreciable quantities of agriculture return flows into the southeastern part of the Delta since 1977, creating a large, brackish marsh (Cienega de Santa Clara). Local agricultural return flows in Mexico have created smaller wetlands in the Delta.

   It is this unique interplay of water, sediment, and vegetation, both prehistoric and contemporary, unregulated for the majority of it's existence and modified in striking ways during the last century, that lures geographers, hydrologists, and biologists to this incredible living laboratory.