Using Town Lake 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. This means that Tempe does not use groundwater in excess of what is naturally or artificially recharged back into the aquifers, preserving that water for years when droughts do occur and there is a shortage of surface water supplies. Tempe is committed to continuing this strategy in keeping with the goals of the Groundwater Management Act passed by the Arizona Legislature in 1980.Tempe has used reclaimed water (treated wastewater) to replace evaporation from the lake through water exchanges with the Salt River Project. This strategy is one component of Temp's extensive efforts in water reuse and water use efficiency.

Tempe is now evaluating a reclaimed water pipeline that will deliver high quality reclaimed water supplies to the lake for supplemental evaporation replacement requirements in the future. The reclaimed water delivered to the lake may also be used to meet water demands to sites adjacent to the lake, such as irrigation of the Tempe Rolling Hills Golf Course, the ASU Karsten Golf Course, and riparian habitat projects.

Lake Seepage Containment
The Tempe Town Lake was designed to minimize water lost due to channel seepage. The seepage containment design has three components:

• Cement slurry cut-off walls tied into subsurface bedrock along the western 1/2 of the lake

• Ten shallow seepage recovery wells along the eastern 1/2 of the lake, capturing channel seepage and re-circulating it back to the lake

• Clay liner throughout the eastern 1/2 of the lake channel to minimize the water required for the initial lake fill

Replacement water – Water to keep Tempe Town Lake at its normal operating level has historically been delivered from three sources: the Central Arizona Project canal system, water exchanges with Salt River Project, and recovered long-term storage credits.

• Central Arizona Project: Evaporation replacement water from Tempe's Central Arizona Project (CAP) water contracts. Tempe has CAP water contracts for municipal supply (CAP M & I contract), excess CAP water, and CAP recharge water contracts. Excess CAP water delivered through the SRP canal system was used for the initial lake filling in 1999, and was used occasionally for evaporation make up water to the lake in 2000 through 2005.

• Water Exchanges: The City of Tempe supplies reclaimed water from the Kyrene Reclamation Plant to the Salt River Project’s Kyrene Generating Station for power plant cooling and industrial uses. This water can be exchanged for SRP canal water for the lake, and was the primary evaporation make up water supply between 2002 and 2004. Water exchanges are an economical alternative and allow for water supplies to be delivered through existing facilities.

• Long-Term Storage Credits: Although rarely used, the City of Tempe can use recovered long-term storage credits as a third option for evaporation replacement water in Tempe Town Lake. CAP water or reclaimed water is recharged to groundwater aquifers to establish long-term storage credits that can be recovered through wells for later use.

Replacement Water (current conditions)
Heavy winter storm runoff from the Salt/Verde River watershed in 2004 and 2005 has changed how the Tempe Town Lake is operated over the past two years. SRP released or spilled water from full reservoirs on the Verde River following a series of winter storms in late December, 2004. This water flowed over the Granite Reef Diversion Dam and down through the lower Salt River. The water flowed through the Tempe Town Lake for over three months, requiring the inflatable dams to be lowered on several occasions.

After these flows ceased in April, 2005, saturated conditions in the channel upstream of Town Lake remained. Adding to this condition are a number of water discharges to the Salt River channel upstream of the lake. These included reclaimed water discharged by the Mesa Northwest Reclamation Plant, several ADOT storm drains for the 101 and 202 freeways, gravel pit de-watering, SRP's Tempe Canal drain, the Indian Bend Wash and several other outfalls and discharge locations. The saturated channel conditions and addition of these other water sources to the Salt River upstream of the lake have resulted in intermittent periods of significant flow through the lake and downstream. The magnitude of these flows has regularly  been greater than any water lost to evaporation. The result is that Tempe has not delivered any evaporation make-up water supplies to the lake since May, 2005.

Flood Control
The design of the Tempe Town Lake has incorporated a number of flood control features. 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, and a rock gabion upper levee designed to contain the 500-year flood event.

The inflatable rubber dams are designed with an automated control system that adjusts the dam's pressure and height to pass increasing amounts of floodwater down the channel. At flows above 36,000 cubic feet per second (cfs) the dams are designed to be fully deflated to pass all floodwater flows down the channel. During the floodwater flows of 2004-2005 the smaller upper dams were deflated throughout the entire floodwater event, and the larger downstream dams were deflated during periods in February, 2005, when flows exceeded 30,000 cfs.