Why is Utah Lake so low, when the upstream reservoirs (Deer Creek and Jordanelle) seem to have lots of water? I turned to Greg Beckstrom, Division Director for Public Services, for an explanation that the rest of us could understand. Below is his response.
A complete answer to this question can be very complicated, but it ultimately comes down to balancing long established legal water rights with some practical considerations. Water is a valuable resource in a desert environment. Those who are responsible for managing this limited resource attempt to do so in a manner that economically maximizes the total amount available to all those who have rights to its beneficial use.
It is helpful to understand the relative size of the bodies of water along the Provo River. The combined storage capacity of the upstream reservoirs is only half the volume of Utah Lake at normal (full) elevations:
- Jordanelle – 310 thousand acre feet
- Deer Creek – 150 thousand acre feet
- Utah Lake – 900 thousand acre feet
An acre foot is equal to an acre of ground covered by a foot of water. Most of the water stored in Deer Creek Reservoir is “imported” (transferred) from the Weber River and Duchesne River basins.
Evaporation is a major concern in managing water storage, particularly in periods of drought. While Utah Lake can hold twice the volume of the upstream reservoirs, it has almost 15 times the total surface area, due to its shallow depth. The greater surface area, combined with warmer water temperatures, result in a much higher rate of evaporation from Utah Lake, than from the upstream reservoirs. In hot summer months this can be 2000 acre feet/day of “lost” water. Total diversions from Utah Lake for irrigation and other uses are 1000 acre feet/day. As a comparison, total water consumption in Provo City on a hot summer day can be 170 acre feet.
As population growth continues along the Wasatch Front, use of Provo River water transitions from agricultural uses to municipal uses. The cost of treating water from the reservoirs to meet drinking water standards is much less expensive than treating Utah Lake water, due to its higher level of salts and silts.
These factors result in Utah Lake getting quite low during periods of drought, such as has been experienced the last five years. Most of the water stored in the upstream reservoirs can do little to help Utah Lake. It would either be lost to evaporation, or never be imported from other river basins in the first place.