Completed in 1966, the Glen Canyon Dam holds back the Colorado River to flood Glen Canyon, forming Lake Powell just upstream from Horseshoe Bend. Looking at the height of white bathtub ring, the water level doesn’t look down much, but a park ranger told me that the reservoir had risen to 55% full after the past wet winter.
Lake Powell loses water to evaporation and leakage; I wondered if the water loss is significant. Running the numbers, Lake Powell loses enough water to supply over half (57%) the people in the San Francisco Bay Area with water — every year. After weathering years of drought, the water loss from Lake Powell is very significant.
Just downstream from the dam, Glen Canyon Bridge “has an overall length of 1,271 feet (387 m) with a deck 700 feet (210 m) above the river, making it the one of the highest bridges in the United States. The bridge was the highest arch bridge in the world when completed in 1959.”
The green on the canyon wall looks like a canyon seep. Is the canyon seep and evaporation loss significant? I’m surprised by the numbers, so I’ve included my calculations so others can check this.
From wikipedia, “According to USBR data for water year 2015 (a year when Lake Powell did not experience a significant overall gain or loss in volume), Lake Powell lost a total of 368,000 acre feet (0.454 km3) to evaporation and only 8,000 acre feet (0.0099 km3) to leakage.” Lake Powell loses loses 376,000 acre-feet of water per year.
I calculated how many people would be supplied with one acre-foot of water in a year. Our local water district says “One person uses, on average, 75 gallons of water a day in Silicon Valley”. Wikipedia says an acre-foot is about 326,000 gallons. First, one acre-feet of water supplies 11.9 people for a year:
1 acre-foot (AF) = 326,000 gallons = 326,000 gallons / 75 gallons/person-day = 4350 person-days = 4350 person-days / 365 days/year = 11.9 person-years
Then, Lake Powell loses enough water to supply 4.38 million people with water for a year:
Lake Powell's water loss = evaporation loss + leakage = 368,000 AF + 8,000 AF = 376,000 AF x 11.9 person-years/AF = 4.47 million person-years of water
4.47 million is a lot of people, but it’s hard to relate to. Per wikipedia, the San Francisco Bay Area had “approximately 7.68 million people” in 2016. The water lost from Lake Powell would supply over half (58%) of the bay area’s population with water for a year. And this happens every year, for a reservoir that’s half full.
Lake Powell's water loss = 4.47 million person-years of water = 4.47 million person-years of water / 7.68 million people in the SF Bay Area = supplies 57% of the Bay Area's population for a year
Lake Powell is very costly in terms of annual water loss. Does Lake Powell provide needed storage? Lake Mead is downstream from Lake Powell, so we can examine storage for both. Here’s the data for August 18, 2017.
|Reservoir||Water stored (MAF)||Capacity (MAF)|
Currently, Lake Mead could store all the water in both reservoirs, but Lake Mead is currently less than 40% full. Las Vegas draws its water from Lake Mead, and low water levels threatened to cut off Las Vegas’ water supply. The original intake for Las Vegas was built in 1971, five years after the Glen Canyon Dam was completed. Since then, low water levels for Lake Mead required construction of successively lower intakes in 2000 and 2015. The 2015 intake valve cost $817 million. It seems that draining much of Lake Powell would have avoided this expensive fix and reduced Lake Powell’s evaporation loss. But here in the American west, water is for fighting over.
The Colorado River water at Horseshoe Bend is clear because the heavy silt load of the Colorado River (as we saw near Moab) is trapped by the Glen Canyon Dam.