Town Lake Water Quality

It is through prudent management and wise use of available water resources that a project like the Tempe Town Lake on the Rio Salado is possible.

Water Quality History

2012 updated 9/24/2012

2009

2008

2006

2005

2004

2003

Town Lake Water Supply, Seepage Containment/Recovery Systems and Flood Control Features

Intitial Fill

Tempe began the initial fill of the 220 surface-acre lake on June 2, 1999, and declared it officially full on July 14, 1999. The City secured water for the initial fill of Town Lake from the Central Arizona Project (CAP), which delivers Colorado River water to Central Arizona as part of Arizona’s Colorado River allocation. This water was delivered through the Salt River Project Canal System, which is connected to the CAP Canal System near the Salt River Project’s Granite Reef Dam east of Tempe.

Seepage Containment

The City of Tempe utilizes a three-tier system of seepage containment and recovery technologies in order to minimize the amount of water lost to channel seepage at Town Lake. The reach of the Salt River through Tempe contains flood control channel levees constructed of cement stabilized alluvium along the banks of the river. These levees form a low permeability barrier around the sides of the lake that restricts the lateral loss of water from the lake. Along the 2 miles of Tempe Town Lake this levee was covered with a structural and aesthetically attractive lake edge. Below the surface of the levees an underground cement-clay cutoff wall connects to shallow bedrock around the western perimeter of the lake, preventing loss of water to lateral seepage below the river channel. On the eastern end of the lake, where bedrock is much deeper, a system of clay and concrete liners was installed onto the Salt River channel in order to minimize seepage losses during the initial fill period. Additionally, 10 seepage recovery wells were installed that ring the eastern ½ of the lake. The seepage recovery well system recirculates almost 100 percent of any water that is lost through Salt River channel seepage along this reach of the river directly back into the lake.

Flood Control Features 

Tempe Town Lake is part of the Salt River channel and as such, is subject to receiving water from upstream. When snow melts from the north or rains fall across our state, water is released from the SRP dam systems into the lower Salt River and then through the Tempe Town Lake.

The dam system that forms the Town Lake was designed to handle a wide range of water flows and occasional large floodwater events on the Salt River and Indian Bend Wash. Prior to construction of the lake the Salt River was channelized and deepened to pass through the maximum expected 500-year flood event in this reach of the river. Two levee systems were also constructed: A cement stabilized alluvium lower levee that carries the 100-year flood event (approximately 180,000 cfs), and a rock gabion upper levee designed to contain the 500-year flood event (approximately 250,000 cfs).

Town Lake utilizes eight rubber bladders that form two inflatable dams segments that contain the water on the east and west ends of the lake. These dams can be rapidly deflated or inflated in 45 minutes or less to release flood waters flowing down the Salt River, or to capture and store the tail end of flood water flows in the lake. Water that is released or passed through the lake during storms can be replaced with the tail end of local flood waters, or by flood waters released or spilled from full reservoirs upstream on the Salt and Verde Rivers.

Pictured above: An example of a movable crest steel gate dam, which has been selected as the future Town Lake downstream dam system

Tempe plans to replace the inflatable rubber dams with a movable crest steel gate system in 2015. The city is moving to the steel gate technology because the harsh desert environment has caused excessive aging of the rubber dams and has shortened their useful life.

Here is a general guide to understanding water flows to Town Lake:
River flows of less than 10,000 cubic feet per second (cfs) normally do not result in any change to standard operations at Town Lake. The dam control system automatically adjusts to increasing discharge rates in the river to maintain a constant lake elevation. If flows exceed 10,000 cfs, the City of Tempe would expect some debris to pass and would address that situation. At river flow rates between 10,000 cfs to 30,000 cfs, the dam control system will continue to maintain a relatively constant lake level while adjusting to pass increasing discharge rates. When flows exceed 30,000 cfs, dams are lowered partially to safely pass through flood flows. At least one or two dam segments are lowered completely when flows exceed 36,000 cfs. As the flood peak passes and flows drop, the inflatable dam segments are completely re-inflated when flows drop to 10,000 cfs or less, capturing the tail end of the flood event to re-fill the lake. 
The Town Lake dam system in Tempe is rated to safely pass the 200-year flood event flow rate of approximately 210,000 cubic feet per second (cfs).

The gauges at Priest Dr. and Curry Rd. account for the water passing through Tempe Town Lake. Anyone can monitor the water flow in current time by clicking on Salt River at Priest Dr. and Indian Bend Wash at Curry Rd.

126,000 cfs

December, 1978   

Replacement Water Supplies

Evaporation losses at the lake during the first five years of operation were replaced by using a portion of Tempe’s CAP Colorado River water, Salt River water from Tempe’s New Conservation Storage capacity at Modified Roosevelt Dam, or water exchanges utilizing Tempe’s reclaimed water supplies. All evaporation replacement water supplies that were needed for lake operations during this period were delivered through the SRP Canal System.
In late 2004 through early 2005 the lake received its first sustained flood water flows from a series of winter storms on the Salt and Verde River watersheds. Runoff from the storms filled reservoirs operated by the Salt River Project (SRP) upstream on the Salt and Verde Rivers, requiring releases or spills from these reservoirs over Granite Reef Dam and down the lower Salt River. The flow through Town Lake peaked at over 44,000 cubic feet per second and continued at various rates through April 2005. The Town Lake inflatable dams operated according to design during this period, releasing and passing through flood waters during peak flows and recapturing water at the tail end of the flood event.
The 2004-2005 flood event and subsequent flood events along the lower Salt River had a profound effect on Town Lake water operations and lake water management practices. Prior to this period the channel upstream of Town Lake was generally dry and the lake received little to no inflow from the ephemeral Salt River channel. All water required to maintain lake levels was delivered by outside sources through the SRP canal system or from Tempe’s seepage recovery wells. Since this flood event in 2005, and subsequent flood water periods through 2010, the river bed has remained saturated and the lake receives almost constant inflow from the various water sources contributing to the Salt River upstream of Tempe. These variable inflows from upstream water sources now constitute the single largest source of water for Tempe Town Lake each year since 2005.

Town Lake has been hailed as a model to forward thinking, progressive planning and efficient water management.

Using Water Efficiently

The City of Tempe relies on renewable and sustainable water supplies, with a diverse mix of water resources available to meet the needs of our water service area. Renewable and sustainable water supplies means surface water from the Salt, Verde and Colorado Rivers, “safe-yield “ groundwater and reclaimed water supplies. Tempe does not use groundwater in excess of what is naturally or artificially recharged back into the aquifers (safe-yield), preserving that groundwater resources for supplemental water supply during periods of drought and as a back-up for Tempe’s surface water treatment plants. Tempe is committed to continuing its reliance on renewable and sustainable water supplies in keeping with the goals of the Arizona Groundwater Management Act passed by the Arizona Legislature in 1980.
Tempe has developed the infrastructure and water supplies necessary to meet the needs of a growing population. Using a combination of technology and innovation the City has developed a plan of operation for the Tempe Town Lake that minimizes the amount of water lost to seepage through the use of seepage control and recovery systems, and utilizes pass-through flows in the Salt River to reduce or eliminate the need for evaporation replacement water supplies.