There were 18,671 upriver summer steelhead counted climbing the dam's fish ladders that day, which fisheries managers believed was the highest daily total ever recorded since the construction of Bonneville Dam was completed in 1938.

The record lasted only one day, according to Joe Hymer of the Washington Department of Fish and Wildlife. There were 28,314 steelhead counted at Bonneville Dam Wednesday.

"Today's count may be even higher -- 1,700 fish were counted in a single hour on the Washington side this morning," Hymer said Thursday afternoon of the Washington shore fish counting window.

And indeed it was. Hymer said the steelhead count at Bonneville rose to an incredible 34,054 Thursday.

The Tuesday number brought suspicions that "someone had fat fingers" and had punched in an inflated count, Hymer said. But the tally was verified with colleagues, as were the succeeding counts. The WDFW runs the fish counting program at the dams, which are operated by the Corps.

The counts are all the highest "at least back to '84 and probably longer than that," said Stuart Ellis, a Columbia River Inter-Tribal Fish Commission biologist. Records in some years before that are not complete, but Ellis and Hymer said a quick search of the available data produced nothing that could top this week's counts.

Not even 2001's record upriver steelhead return produced a daily count so high. There were several counts that year of more than 10,000, 11,000, 12,000 and even 13,000, and one of more than 14,000. More than 630,000 steelhead passed the dam that year. But none the daily counts came close to 18,000.

The 2008 return of upriver summer steelhead to Bonneville Dam was 355,000 fish and this year's preseason forecast is a return to Bonneville Dam of 351,800 upriver fish. The forecast for wild fish totals 89,900 steelhead and includes a strong Group B component bound for Idaho rivers.

Overall, the 2009 forecast is similar (99 percent) to the recent 10-year average of 357,000 fish, according to the "2009 Fall Joint Staff Report" compiled by the Oregon and Washington department of fish and wildlife staffs.

If this week's counts hold true, already more than 258,000 steelhead have crossed over Bonneville. That's not far behind 2001, when 363,570 steelhead had been counted at Bonneville through Aug. 13.

Both Ellis and Hymer say that the huge surge in steelhead passage may have been, at least in part, due to the weather. July ended on a heat wave that pulled the month's average temperatures up to from 1 to 6 degrees above normal across the Columbia basin.

That, in turn, boosted river water temperatures to nearly 75 degrees at Bonneville and at gauges downstream. That's well above a steelhead's comfort level. Temperatures above 68 degrees are known to cause physical stress for salmon and steelhead.

When such warm water is encountered, the fish may search out cooler water, such as at the mouths of tributaries.

"We've seen it in the past with chinook and to some degree with steelhead," Hymer said.

So, with an early August cooling of the air temperatures and the water, a trigger point may have been reached.

"All of the fish that didn't pass for the past week and half" apparently decided it was time to continue their spawning trip, Ellis said.

The Bonneville steelhead counts did slowly increase through July to 8,090 on July 23, dipped a bit to the 6,000-fish range for a few days and then spiked a little again on July 31 to 8,616.

The counts then shrunk with each passing day to a low of 1,407 on Aug. 6 before starting to climb the ladder again. Monday's count was 8,229.

Ellis said that it is too early to tell whether the huge counts are simply the fish playing catch-up, or that the run may be bigger than forecast.

Before the Tuesday-Thursday stampede, a total of 176,483 steelhead had passed Bonneville but only 67,329 had made it over The Dalles Dam, the next hydro project upriver.

The delay there is typical, Hymer said. With the peak of the run timed at the peak of the warm season, many of the fish decide to dawdle at the mouths of tributaries to the Bonneville pool such as the Wind, White Salmon, and Klicktat rivers, and at Drano Lake, before resuming their journey around Labor Day when the weather has cooled.

"Fishing has been good in those areas," Hymer said.

The upriver summer steelhead include hatchery and wild fish that pass Bonneville Dam during April through October of each year. Fish passing April through June are considered Skamania stock steelhead destined mainly for tributaries within Bonneville Pool, and fish passing during July through October are categorized as Group A index or Group B index fish, based on fork length with fish measuring longer than 78 centimeters (30.7 inches) considered Group B fish.

The larger group B steelhead primarily return to tributaries in the Salmon and Clearwater rivers in Idaho, while Group A steelhead return to tributaries throughout the Columbia and Snake basins.