Geologists are going high-tech and reaching deep into the past in efforts to unveil just how much rock and muck is stuck behind Gold Ray Dam, the first step toward its possible removal from the Rogue River.

Geologists are going high-tech and reaching deep into the past in efforts to unveil just how much rock and muck is stuck behind Gold Ray Dam, the first step toward its possible removal from the Rogue River.

Sonar pulses from a torpedo-like contraption towed behind a powerboat will help a team of Southern Oregon University researchers develop the depth and breadth of the sediment behind the 104-year-old dam.

That information, along with water-depth readings and other data, will be matched against Peter Britt photographs dating back 120 years to help gauge just how far rock, silt and mud have backed up behind the Gold Hill-area dam over the decades.

"The oldest photos will give us an idea of what it looked like before the dam was there," said Bill Elliott, an SOU geology professor hired for the study by Jackson County, which owns the dam.

"For something like this, you use all the data you can possibly get hold of," Elliott said.

The work, which began Friday and will run through Monday, launches themuch-awaited sediment study required by the U.S. Army Corps of Engineers.

The study is a precursor before removal, by as early as 2012, of the last great barrier to Rogue River fish and boats between the Cascades and the sea.

"We're actually starting to do the field work," said Steve Mason, a contract biologist hired by Jackson County to oversee the project. "It's exciting to actually be collecting the data. It's a real concrete step forward."

Once a power-generation facility that funneled electricity to Medford 104 years ago, Gold Ray Dam has not served as anything but a place to count salmon and steelhead since the powerhouse was decommissioned in 1972.

The dam and surrounding land was sold to Jackson County with the intent upon creating a park. But it remains a civil liability.

The dam is considered one of the 10 worst artificial fish-passage impediments in Oregon, according to the Oregon Department of Fish and Wildlife.

The dam's fish ladder at times causes upstream migration blockages, and salmon and steelhead smolts heading downstream can plunge over the dam's 35-foot drop while finning toward the ocean.

With Savage Rapids Dam set for removal next year and the Gold Hill diversion dam removed last summer, Gold Ray Dam remains the last major impediment between Lost Creek Lake and the Pacific.

Before the dam could be removed or notched, the county must assess the sediment, and that work began Friday when Elliott motored around the Gold Ray reservoir taking GPS readings and water-depth measurements.

The heavy work was scheduled to begin today when a boat will tow the sonar machine around the slack water.

Its pulses will log the thickness and distribution of the sediment, Elliott said.

The soundings will then be added to an array of data that includes aerial photographs as well as historical photographs taken by Jacksonville's Peter Britt and others from a single vantage point before and during the dam's construction.

A sediment map could be completed by mid-December, said Elliott, who is working with fellow SOU Professor Eric Dittmer and two undergraduate students on the study.

Mason said the map will help determine where to take core samples of the sediment and how many core holes to dig.

The coring will reveal amounts of fine sediments, gravel and cobbles and show any contaminants, such as mercury, or minerals, such as gold, Mason said.

That coring could be done as early as mid-June, and the county has yet to seek a contract for that work.

Funding for the project so far has come from a $100,000 Ecotrust grant the county received last summer.

Reach reporter Mark Freeman at 776-4470, or e-mail mfreeman@mailtribune.com.