There was a proposal to do this to the California aqueduct, but it turns out they don’t lose much water from evaporation and it would be cheaper to put them on land.
https://green.blogs.nytimes.com/2011/04/20/could-the-califor...
https://green.blogs.nytimes.com/2011/04/20/could-the-califor...
According to that paper, 700 miles of canals lose 9300 acre feet of water per day. They say the price per acre feet (in 2015) was $107. Almost a million dollars per day is lost according to that paper. And California hasn't been cooling down since 2015.
It looks like the paper looked at one section of the canal and extrapolated the evaporation loss to the rest of the canal system in California.
*this paper seems to be from a class at the university. Just using it as an example that it might be more evaporation than stated above
Sorry you're getting buried.