Frequently-Asked Questions About Horsetooth Reservoir Dams

1. Are the dams at Horsetooth Reservoir safe?

All the dams are safe, and are not about to fail. Continued frequent and extensive monitoring indicates that all four dams are performing safely. No unusual seepage has been detected through any of the dams; foundation seepage is a concern only at Horsetooth Dam. The modifications will ensure the continued safe operations of the dams into the future. They provide defensive measures against conditions that could arise in the future that would threaten the large population living immediately below the dams. Reclamation believes that these proactive measures are warranted to ensure continued public trust and confidence in the facilities.

2. If the dams are safe, what’s being done to them?

Because so many people live downstream from these dams, they must be brought to the current state of the art of dam design and construction. Three actions are being performed to modernize all four dams. First, layers of sand and gravel are being added to each dam, to allow normal seepage through the dams while preventing erosion of the dams themselves ("piping").
Second, a berm of additional material is being added to the base of each dam, to stabilize it in the case of an earthquake.
Free iphone for you Finally, an impervious blanket will be added at Horsetooth Dam only. The blanket will cap about 4 _ acres with a plastic liner covered with clay. It will prevent water from entering the exposed limestone and seeping through the rock beneath the dam.

3. What about the sinkholes that were discovered?

A single seepage entry location (sometimes referred to as a sinkhole) was revealed at Horsetooth Dam in late September 2000 as the water level dropped. Its location was directly in line with the limestone geology, confirming Reclamation’s predictions that the limestone was a major culprit in allowing water to seep under Horsetooth Dam. Sonar scans in 1999 had located this and other possible "anomalies." For a few weeks in the fall of 2000, the pool at Horsetooth Dam was drained to "dead storage" to treat the entry location and to identify the other anomalies. The entry was plugged with concrete and capped with plastic and clay before the reservoir began rising again. Analysis of geologic samples and of data gathered after sealing this entry allowed Reclamation to treat the foundation seepage with an upstream blanket, rather than the concrete cutoff wall originally planned. No other sinkholes were found near the dam. The other anomalies were identified as erosion and deposits left from original construction.

4. Why does the water level have to be restricted during construction? What impact will the restriction have on water supply, power, and recreation?

During a portion of the construction, the dams are temporarily less stable than usual. Therefore, a lower water surface is required to reduce the water pressure against the dams. After extensive investigation and analysis by Reclamation, a water level restriction of 5360 feet was determined. This elevation reduces the water pressure sufficiently, while still providing enough water in the reservoir to serve needs. At 5360, Horsetooth will be approximately a third full. Water levels will still fluctuate, but not rise above 5360 until construction is nearly complete.

During construction, boating will be possible, but limited, from the late spring into the summer. By working together, Larimer County Parks and Open Lands and Reclamation extended the Swim Beach boat ramp and constructed a new boat ramp specifically for low water levels so boaters will still have water access.

5. What’s the schedule for construction?

Phase 1 is construction of a filter, drain, and berm at Horsetooth Dam. Construction on this phase began in March 2001, and is scheduled to be complete in the summer of 2002. Phase 2 is construction of filters, drains, and berms at each of the three east-facing dams. It began in the October 2001 and is scheduled for completion in the late summer of 2003. Reclamation plans to "relax" the water level restriction during the last several months of construction. Phase 3 will be construction of an upstream blanket at Horsetooth Dam. Construction is planned for fall 2002. Closeout activities will include landscape rehabilitation and road re-paving.

6. How much will this work cost, and who pays for it?

The estimated cost for the entire project is $105 million. That includes the cost of exploration, design, and construction. Under the 1984 Safety of Dams Act, the Federal government pays 85% of the cost. The remaining 15% is split among the project beneficiaries. In this case, water users (the Northern Colorado Water Conservancy District) pay 7.5% and power customers pay 7.5%

7.What are the roles of the Bureau of Reclamation and the Northern Colorado Water Conservancy District?

Horsetooth Reservoir is a key part of the Colorado – Big Thompson Project (C-BT), built and owned by the Bureau of Reclamation. Consequently, Reclamation is responsible for ensuring that the dams meet current standards for public safety. The Northern Colorado Water Conservancy District was formed to repay the costs of the C-BT Project. They distribute water to irrigators and municipal and industrial water users. The District also operates and maintains the Horsetooth Reservoir dams, along with other C-BT facilities.