One of the earliest water projects in the Colorado River basin was the Grand Ditch, a diversion canal that sends water from the Never Summer Mountains, which would naturally have drained into the headwaters of the Colorado River, to bolster supplies in Colorado's Front Range Urban Corridor. Constructed primarily by Japanese and Mexican laborers, the ditch was considered an engineering marvel when completed in 1890, delivering across the Continental Divide each year. Because roughly 75 percent of Colorado's precipitation falls west of the Rocky Mountains while 80 percent of the population lives east of the range, more of these interbasin water transfers, locally known as transmountain diversions, followed. While first envisioned in the late 19th century, construction on the Colorado-Big Thompson Project (C-BT) did not begin until the 1930s. The C-BT now delivers more than 11 times the Grand Ditch's flow from the Colorado River watershed to cities along the Front Range.