Morelos Dam was built in 1950 pursuant to the 1944 Water Treaty between the United States and Mexico, which allocated the Colorado River's flow between the two nations. The treaty guaranteed Mexico 1.5 million acre-feet per year — water that would be diverted from the river into the Mexican agricultural systems of the Mexicali Valley and Sonora. Morelos Dam was the infrastructure that made that diversion possible.

The dam sits in a peculiar geographic and jurisdictional position: its eastern half lies in the United States, its western half in Mexico, but Mexico operates and maintains it. The structure itself is unimposing — a low diversion dam rather than the towering concrete arches of Hoover or Glen Canyon — but its effect on the river is total. By the time the Colorado has passed every upstream allocation, every municipal draw, every agricultural diversion on the US side, Morelos Dam captures what remains and sends it west and south into Mexican farmland.

Aldo Leopold wrote about the Colorado River delta in 1922, when he canoed through it. He described 'a milk-and-honey wilderness' and 'awesome jungles,' a place of extraordinary biological richness fed by the river's seasonal floods — cottonwoods, willows, birds beyond counting, the dense growth of a desert made lush by water.

By 1960, the delta was largely dead. The upstream dams had captured the floods. The upstream diversions had captured the flow. The 'awesome jungles' had dried out, the cottonwoods had died, the wildlife had moved or vanished. What the river delivered to the delta was, in wet years, a trickle and in dry years, nothing at all — sometimes not even reaching the Gulf of California.

Chris McCandless, who became famous for his fatal attempt to live alone in the Alaskan wilderness, kayaked the Colorado River in 1991 on his way to Mexico, passing through this zone on his journey south. He made it to the Gulf of California. The river he paddled was already deeply diminished.

In 2014, the United States and Mexico agreed to an experiment. For eight weeks, beginning in March, they would release a controlled flood down the Colorado — enough water to flow through Morelos Dam and continue down the dry riverbed toward the sea. The goal was to test whether a deliberate pulse of water could begin to restore the ecological conditions that Aldo Leopold had described ninety years earlier.

The release — 105,392 acre-feet of water — traveled down the parched channel and reached the Gulf of California for the first time in sixteen years. Within days, vegetation that had been dormant in the dried channel began to respond. Birds arrived. Scientists documented the movement of the water and its effects with the attention of people who understood they were watching something that might not happen again soon.

The experiment ended. The 12,000-acre Ciénega de Santa Clara wetland, created accidentally during construction of the US desalting plant's MODE canal and now one of the most significant wetlands in the Sonoran Desert, received some of this water and continued to serve as an important refuge for wildlife that had lost the delta habitat.

Morelos Dam processes the mathematics of over-allocation. The Colorado River is legally committed to delivering more water than it carries in an average year — a situation created by the 1922 Colorado River Compact, which divided the river during an unusually wet period. Every dam and diversion above Morelos represents a prior claim. By the time the water reaches this final structure, the accounting has usually run out.

The dam itself, low and unspectacular, spans the river just south of the border crossing near Los Algodones, Mexico and Andrade, California. From above, it is visible as a line across the water — a modest structure with an immodest consequence. The river that enters it is the Colorado. What emerges to flow south toward the delta is, most of the year, not enough to make the trip.