Most water in Utah's streams comes from melting snow. The sun heats up this water as it flows down stream. Even though the air temperature gets warmer throughout the summer, the water in these high streams often stays very cold.
Shade from trees along the river's edge helps keep the water cool.
Streams without these shady plants get more sunshine and heat up faster.
Try this out: stand in the sun for one minute and then move under the shade of tree. The only difference in how you feel is how much sunlight is shining on you.