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Updated estimate of sediment in former Elwha lakes

January 3rd, 2013 - 1:32pm

(Port Angeles) -- With the new year, there's an updated estimate of the total volume of sediment stored in the former Lake Mills and Lake Aldwell.

Barb Maynes of Olympic National Park says members of the sediment team recently discovered a long-hidden error in the first recorded survey of the Lake Mills area.

Completed in 1917, ten years before Lake Mills was flooded, this survey contained mislabeled elevations - the true elevations are actually 20 feet lower than the labels indicate.

Over the years, the 1917 survey, including the 20-foot error, served as the basis for all subsequent maps.

After 85 years and some careful sleuthing in the Library of Congress, Bureau of Reclamation engineers located and corrected the mistake.

The story doesn't end there, however, as the elevations have long been used to estimate the total volume of sediment contained within Lake Mills.

Lowering the elevations by 20 vertical feet means that Lake Mills (and its sediment) are actually 20 feet deeper than previously estimated.

The updated estimate of sediment in Lake Mills is approximately 28-million cubic yards.

Pre-dam maps and surveys are not available for Lake Aldwell, making that estimate even more challenging.

The actual sediment volume in Lake Aldwell is becoming more apparent as more of the predam surface is exposed by river erosion.

Engineers estimate that the Lake Aldwell sediment volume may be closer to 6-million cubic yards in that reservoir bed.

A total of 34-million cubic yards is difficult to visualize, but is enough to cover a football field with a 5-and-a-half-mile deep layer of silt, sand, gravel and cobbles.

Although the change in estimated volume is significant, it is not expected to greatly influence either how long or how heavy the river's sediment loads will be.

The rate of dam removal, controlled in response to rainfall, floods, spring melt and other factors influences the rate and amount of sediment erosion.

Sediment impacts remain unchanged and well within the parameters of the existing water treatment facilities and other project mitigations.

Dam removal is still anticipated to be complete well within the contract period.

The discovery illustrates the challenge of mapping an unseen landscape and estimating the quantity of buried sediments.

Even with today's satellite and GPS-linked mapping equipment, the floor of the Elwha Valley in Lake Mills and Lake Aldwell hasn't been seen in decades, a key reason why a team of geologists and engineers continue to monitor and evaluate the amount and rate of erosion in the Elwha.

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