Panoramic View of the Silverton Dam's Spillway
Silverton Dam & Reservoir: The FactsDo the dam and reservoir on Silver Creek above Silverton pose an unacceptable risk to the town and the downstream community?
The Dam
The dam, built in 1974 two miles upstream from Silverton, and owned and maintained by the City, is an earth and rock fill embankment with a packed core, 65 feet high and 680 feet across, with a 120-foot wide concrete spillway sixteen feet below the top. A 42-inch pipe at the base allows controlled water release. The reservoir holds more than 436 million gallons (1,300 acre feet).
Water Supply
Silverton's water comes from a dam on Abiqua Creek. Backup water can be drawn from Silver Creek near the city swimming pool and pumped to the water plant. The Silverton Reservoir cannot gravity-feed to the City's water storage facility. Water must be released from the dam to flow downstream to the intake. This backup has not been used. A summer draw down for drinking water would pull toxins and decaying matter off the bottom of the reservoir.
Dam Safety
According to the U.S. Army Corps of Engineers, earthen dams have an average life of fifty years. Many such dams fail in the United States every year. Silverton's dam is described by the Dam Safety Division, Oregon Water Resources Department, as "a high hazard structure in the intermediate size category indicating that a sudden failure of the embankment could result in loss of life." The city has failed to implement repeated recommendations by the state, including a log boom to prevent spillway blockage.
Earthquakes
The Marion County Natural Hazards Mitigation Plan indicates that the major earthquake fault band across the county goes directly through the dam area and is a high hazard zone. Localized earthquakes occurred in spring 1993 and autumn 1999. The predicted subduction earthquake off the Oregon Coast (estimated as high as 9 on the Richter Scale) would likely cause the Silverton Dam to fail.
Changing Weather Patterns
Silver Creek flooded in 1996, threatening buildings on James Street and along Silver Creek. Logs and debris threatened bridges. The base of the dam was significantly eroded. In November 2006 multi-day rain resulted in one of the wettest periods ever seen in the Northwest, setting new rainfall records along Silver Creek. La Niņa weather patterns are expected to continue for some years, producing exceptional rainfall and significant floods. Once one inch per day, there are now 3-inch per day events. Recent storms exceeded estimates in California, Washington and Northwestern Oregon (see Mount Hood floods.) Our dam would not withstand a direct hit by similar storms.
Flood Threat
In 2000 the City Council commissioned a dam break analysis, which described two catastrophic scenarios (See 97381.com/dam):
- If the spillway could not keep up with the flow of water into the reservoir, the rising volume of water behind the structure, (approaching a billion gallons,) could overtop the dam, eating away the bank until it collapses.
- "Piping," in which small leaks through the dam eat away at the material, could expand, and lead to the dam's collapse. There has been ongoing seepage from the front of the dam from the beginning. Current seepage indicates erosion from within.
| Location | Depth in Feet | Available Minutes | Cu.Ft./Second | Gal/Second |
| Old High School | 15.8 | 52 | 56,000 | 431,000 |
| Silver Gardens James St. | 14.0 | 51 | 56,000 | 431,000 |
| C. Street | 11.1 | 49 | 57,000 | 437,000 |
| Eugene Field School | 7.6 | 41 | 59,000 | 454,000 |
| Water & Main Sts. | 6.0 | 40 | 60,000 | 462,000 |
| City Hall | 8.5 | 40 | 61,000 | 470,000 |
| Central St. Bridge | 8.5 | 28 | 70,000 | 539,000 |
| Ike Mooney Rd. & Water St. | 10.3 | 10 | 84,000 | 647,000 |
| Creek at Quall Rd. | 24.0 | 2 | 102,000 | 785,000 |
City Responsibility
The City of Silverton has failed to implement many of the state's recommendations or to take sufficient precautions against a failure. There is no remote monitoring system and no early warning system for the community. The city lacks an up-to-date emergency plan.
The Emergency Action Plan
An Emergency Action Plan for Silverton must address the rapid and massive evacuations required by dam failure, including detailed plans for the schools. Fully equipped emergency bases should be available outside the flood zone. Neighborhoods should have detailed response plans for escape and shelter. Public Safety and medical personnel must be trained. Disaster recovery steps for a devastated community are part of this plan.
Worth the Risk?
Conditions have changed since the dam was built. Escalating weather and earthquake dangers top the list. Is the dam a realistic backup water supply or a mistake we fail to recognize? Do the unused water supply backup and limited recreation facilities justify the ongoing threat to Silverton families' safety and the town's very existence?
Decide What Risk You Want To Take.