And let them help pay for it proportionate to the percent of the water produced desalinating through evaporation and condensation collecting in it. Modern earth moving equipment could make short work of digging a U.S.-Mexico border control canal from the California mountains to the Gulf of Mexico. Water would be siphoned up from the pacific and or the Sea of Cortez to New Mexico and flow downslope in each direction. Sun shine and solar panels would create power for the pumps. The canals would be covered with a glassy plastic to let the sun wart the water that would be a solar still in effect to make fresh water.
The salt produced would have many uses too. I would think that the millions of gallons of renewable water could alleviate some California farming shortages and the salt trivially supply salmon farming concerns using the freshwater too, built in Colorado and Phoenix (air conditioned) near consumers. Farm grown fish far away from the oceans is a safe way to produce that fine food. It also saves corruption of coastal areas from fish farming derivative effects.
The trans-continental railroad was built by a great leader; U.S. Grant who had the leadership skill-set to get it done. Dwight Eisenhower got the interstate highway system built, started the space program, nuclear power program, Grand Coulee Dam etc. If President Trump could get a water-making border control canal system built powered with sunshine he would indeed rank up their with Grant and Eisenhower. Mexico may want something for its money if it helps pay for the canal-such as water and energy from the solar panels generating electricity. That is only reasonable. Its water and energy share should be proportionate to its finance participation. If the canal can produce 50% of the water of the Colorado River eventually that could be a bargain.