The depression is nearly as deep as Death Valley, but the well-intentioned irrigation project worked for not very long, as someone quoted/noted above. Unusually high seasonal flooding broke through and filled the newly productive irrigated farmland, and the waters absorbed the agriculture chemicals as well as leaching out various salts and crud in the subsoil. For a long period though Salton Sea was a popular spot for Californian and other SW vacationers.
Eventually the waters stank of fish that succumbed to the contaminants and probably the additional human wastes that somehow or other (koff koff) made it into the lake. If a borehole downhill into the Pacific (Gulf of California) were made, and survived the probable small quakes from the weight shift of the water, over time the Salton would turn into a clean inland saltwater sea, but likely destroying any existing freshwater aquifers in the process.
Whoops.