June 9, 2004
(The following is an excerpt from Doug Freed’s book, “Colorado by the Numbers.”)
One-half of 1 percent of Colorado is covered with water. It is the state’s most precious commodity, and most of it is on its way out, flowing ever downhill. As the Highest State, Colorado is the Headwaters State.
Only three rivers, the Green, Little Snake and Cimarron, flow in to Colorado, and that for only short distances. Discounting the short stints of the Green, Little Snake and Cimarron, Colorado receives not one drop of water from another state. Every raindrop, every snowflake that falls on the state is collected in one of our 94 watersheds and sent to the Pacific or the Atlantic Oceans via one of 10 major river drainages.
If only it were that simple.
The people have settled where the water isn’t. Colorado’s Western Slope, the area west of the Continental Divide, accounts for three-quarters of the state’s precipitation, but only two-fifths of the state’s land area, and one-fifth of the population. East of the Continental Divide, the arid Front Range, is home to nearly 80 percent of the state’s population, but only 25 percent of the water.
The answer has been to move it and store it. More than 40 diversion projects move water from one drainage to another — 24 move water from the Western Slope to the Front Range. Nearly 2,000 reservoirs collect what they can — about 8.9 million acre-feet.
In all, about 10 million acre-feet of water leaves the state annually via the 10 major drainages. Approximately 8.5 million acre-feet are promised to downstream users via one of 18 interstate water compacts. Even with 1,900 reservoirs, roughly 1.5 million acre-feet of water belonging to Colorado cannot be captured and flows to other states.
87 percent of the water leaving Colorado does so via the Colorado River and its tributaries. Seven states and Mexico share the cherished moisture. Back home on the Front Range, only ambitious water projects make farming practical and large cities possible.
Flow of Colorado rivers at state line
19 major rivers leave Colorado with an annual average of 10,434,830 acre-feet of water. 87 percent, or 9,097,470 acre-feet, flow west, eventually to the Colorado River and the Pacific Ocean. Only 1,337,360 acre-feet escape to make it to the Gulf of Mexico — 25 percent of that in the Rio Grande, the rest in the Platte and Arkansas River systems.
Most of Colorado’s water is stored in the form of snow. While reservoirs today mitigate raging spring floods, it is estimated that 50 percent of the state’s river volume runs in only two months of each year. About 4.5 million acre-feet are used annually to replenish the 1,900 reservoirs across the state. Agricultural irrigation uses a whopping 11 million acre-feet each year. Another 2 million acre-feet are used for municipalities, industrial uses and fisheries. Colorado residents use roughly one-half of the state’s surface water, leaving the other half for downstream users. Approximately 29 percent of the water used by state residents is groundwater.
Colorado can boast 2,286 naturally occurring lakes. 89 percent of the lakes are above 9,000 feet, pools in the upper reaches of high mountain valleys. The total surface area of Colorado’s lakes is a mere 20,333 acres. The deepest lake is Grand Lake in Grand County at 265 feet. Grand Lake also serves as the headwaters for the Colorado River, and as an important link in the state’s largest transbasin diversion project, the Big Thompson.
Water Storage — Reservoirs
If the rain and snow stopped falling, and the rivers stopped running, Colorado could survive for a time — a very short time — on the 8.9 million acre feet of water collected and stored in its 1,900 reservoirs. A two-year drought would reduce even mighty rivers to a trickle and leave reservoirs empty. Without its reservoirs, Colorado could not support its cities or its agriculture base.
With an estimated one-half of the state’s water flowing during a two-month spring runoff period, reservoirs are vitally important in capturing and holding the spring swell of water. The water is carefully measured, even as it sits as snow on the mountains. Colorado snow, fresh and uncompacted, is generally 90 percent to 95 percent trapped air. Still, it is the primary source of Colorado water and without reservoirs, much of our water would flow out of the state unused.
More than 90 percent of Colorado’s water is used to irrigate crops. Less than 1 percent of the water treated by public water systems is used for drinking and cooking. Approximately 10 percent of water stored in reservoirs is lost to evaporation or conveyance loss.
Blue Mesa is the largest reservoir, Lake Granby is second. Of the 104 reservoirs exceeding 10,000 acre-feet of storage, 65 are dedicated to irrigation, 17 to municipal uses, six to hydro-electric power generation, eight to recreation, five to flood control and one each to domestic and industrial uses.