THE DEPARTMENT OF WATER Resources in the state of Idaho, US, has ordered the Fish Creek reservoir company, the owner of the Fish Creek dam, to undertake complete repairs to nine buttresses and build increased spillway capacity for the dam or to reduce storage.

State authorisation to store water beyond the 2004 irrigation season will depend on a dam inspection and on progress in plans to fix other deteriorated portions such as the arches in the maximum height section. If the company is not able to repair the buttresses soon, it will be required to limit holding capacity by installing an outlet at 20.4m or at the height of completed repairs.

The outlet has been provided as an option to avoid breaching. Other safety measures the owner must take include regular testing of the dam’s emergency warning system with no less than 100% system reliability, and daily visual inspection of the downstream face of the dam once the dam fills to 20.4m.

In its existing condition, the 28m-high dam has been deemed safe to store water only up to 20.4m. The department says repairs must be made or the dam must not hold more than about 5.3Mm3 of water, about one-third of its more than 14.8Mm3 capacity. The concrete dam, completed in the early 1920s, provides irrigation water supply to about 3200ha. A dam inspection in August 2003 revealed continuing deterioration so severe that holes appeared in seven locations through 0.6m-thick concrete buttresses.

Flood plain studies have shown that in the event of a structure failure, about a dozen homes and a state highway would be flooded. Estimated costs to rebuild the dam range from US$7.5M–US$20M.