SPOKANE — Modifications on the cracked Wanapum Dam will allow migrating salmon to pass through the structure starting this week on their journey to spawning grounds, the Grant County Public Utility District said Tuesday.
The utility has extended fish ladders, making them functional even after water was lowered behind the dam to prevent the crack from getting worse.
Spokesman Tom Stredwick said the utility spent $1.5 million to extend fish ladders by 18 feet so salmon can pass through the 8,600-foot wide dam on the Columbia River during their spring migration.
“They will travel up the ladders, slide down the flume, and continue their journey upstream to spawn,” Stredwick said Tuesday.
“These are different conditions, but the fish are used to jumping upstream and downstream,” he said. “These are not outside the norm.”
Divers discovered a 65-foot crack across part of Wanapum Dam’s concrete spillway in February, and workers have lowered the reservoir level by 26 feet to reduce pressure on the dam. That left the upstream exits of the fish ladders high and dry — unusable for the migrating fish.
In addition to extending the fish ladders, the utility this week is starting a program to trap migrating salmon, place them in a tanker truck and drive them around Wanapum Dam, Stredwick said.
The trucking provides insurance in case the modified fish ladders prove difficult for the fish to navigate, Stredwick said. The trucking will also be necessary when the much larger runs of salmon this summer overwhelm the capacity of the fish ladders, he said.
“We don’t want all our eggs in one basket,” Stredwick said.
The utility has set up a fish trap at Priest Rapids Dam, 18 miles south of Wanapum Dam, where it will collect fish, place them in trucks, and drive them around Wanapum Dam to continue their journey up the river to spawning grounds, Stredwick said.
Thousands of spring chinook are already headed up the Columbia River, and about 1,200 to 1,500 spring chinook will soon be passing Wanapum Dam each day. That will climb to a peak of about 25,000 migratory fish per day during the summer run, utility officials have said.
All modifications will have to be removed when the cracked spillway is fixed and the reservoir level is raised to normal, the utility has said.
The cause of the crack remains unknown, but the utility has ruled out four possibilities: seismic activity, foundation settlement, operation of spillway gates, and explosions at the nearby Yakima Training Center operated by the U.S. Army.
Chuck Berrie, assistant general manager of the utility, has said the cause of the crack might be determined as soon as the next two or three weeks.
The crack was detected by divers on Feb. 27, three days after a worker at the dam noticed that the top of a spillway pier had shifted slightly. When the reservoir behind the dam was drawn down by 26 feet, the pressure on the spillway was reduced and the fracture closed.
Six holes have been drilled into the pier and more holes are planned to determine the geometry of the fracture and how far it goes into the pier, the utility said.
The utility said it may be June before a repair plan is formulated.
Wanapum Dam is the Grant County PUD’s largest power producer, capable of generating 1,092 megawatts of electricity, enough to supply nearly 900,000 homes.
The giant dam continues to generate electricity at 50 to 60 percent of its capacity, the utility said.
The crack runs the entire 65-foot width of the concrete base of the dam’s No. 4 spillway pier. The dam has a total of 12 spillways and is located just south of Vantage, where Interstate 90 crosses the Columbia River.