Conservation Corner: Disappearing Summer Steelhead – The Fate of Overshoot Steelhead Seeking Cold Water in the Columbia and Snake Rivers

By: Greg Fitz, Trout Unlimited West Coast Communications Director

Most of the summer steelhead returning to the Columbia Basin each year arrive between June and September. After leaving the Pacific, these fish overwinter in freshwater and eventually spawn the following spring. During their migration, some travel directly to their spawning streams while others spend months holding in the Columbia and Snake mainstems before moving on to their natal waters. But a surprising number of steelhead move up and down the watershed before spawning, often seeking cold water refuges, expending considerable amounts of energy and crossing multiple dams in the process.

Salmon and steelhead that swim past their natal streams are known as “overshoot” fish. Later, when it is time to spawn or water temperatures have improved, these fish will “fall back” to their home waters.

But on tributaries throughout the basin, especially in the mid-Columbia, a substantial percentage of overshoot fish never make it back to their natal streams. Some of these adult steelhead stray to other tributaries and will end up spawning there instead, but many are killed as they attempt to navigate back downstream through the hydropower system.

Map courtesy Trout Unlimited

Tracking Overshoot Steelhead

Overshoot steelhead have long been understood to be a factor among Columbia Basin populations, but a pair of recent studies using tagged adult fish (Richins and Skalski 2018, and Murdoch et al. 2022) have demonstrated the phenomenon is much more extensive than previously thought. This has huge implications for steelhead management and recovery, and the operation of the basin’s hydropower system.

Gary Marston, the Science Advisor for Trout Unlimited’s Wild Steelheader’s United (WSU), wrote in-depth about the studies and their implications in a pair of articles on the WSU blog. This ‘Conservation Corner’ for Swing the Fly is a summary of his longer “Science Friday” posts. (Read Part One and Part Two here.)

While overshooting is believed to be relatively rare in free-flowing rivers, it is pervasive throughout the Columbia Basin, especially within the Snake River watershed. When certain tributaries, or the slow-moving water impounded behind large mainstem dams, become too hot during the warmest parts of the season, many steelhead (and some salmon) are forced to seek out places with adequate water temperatures. For fish destined to tributaries like the Yakima, John Day, Walla Walla, Umatilla, and Tucannon Rivers, that means swimming further upstream to cooler places higher in the basin, though these fish are limited to habitat and tributaries that aren’t completely blocked by barrier dams.

Beyond the need to find cold water, the relationship between dam and fish ladder locations and the mouths of tributary streams also appears to influence overshoot rates. For example, when steelhead need to use fish ladders on the opposite shore of their natal streams, they overshoot at a higher rate, presumably because the large impoundments make it difficult for the fish to smell their home waters.

Steelhead from the John Day River, which is located only 2.5 river miles upstream of the John Day Dam, appear to have the highest average overshoot rate of any studied population. The suite of mechanisms influencing this, such as low flows or factors that interfere with homing, are currently being investigated.

Graphic Courtesy NOAA

Hydropower Impacts on Adult Steelhead Moving Downstream

Among the tagged steelhead, some tributary-specific populations saw over half of the fish overshoot their natal streams. Many of those adult fish never return to their home waters.

The Umatilla, Walla Walla, John Day, and Tucannon Rivers had particularly high overshoot rates. While some of these fish traveled further up the Columbia River, the majority ended up somewhere within the Snake River watershed. In contrast, Yakima River and Snake River steelhead often overshoot into the upper Columbia River, where approximately two out of five of these steelhead never returned to their natal streams.

Studies show that a very small percentage of these disappearing fish, only around 1%, were captured by fisheries. While few fish in the Upper Columbia strayed onto the spawning grounds, many steelhead in the Snake River were later detected as strays on the spawning grounds of tributaries upstream of the dams. The largest number of lost fish came from adult steelhead killed as they attempted to migrate downstream through the basin’s large mainstem hydropower dams.

Like other fish in the basin, overshoot steelhead migrate upstream using fish ladders at the dams. But when the time comes for these adult fish to fall back to their home waters, the smolt passage and flow regimes designed to allow juvenile fish to successfully pass downstream through the dams in the spring and early summer often aren’t available when overshoot steelhead need them.

Adult steelhead travel near the surface of the river as they swim downstream. When they encounter a dam, they would prefer to use spillway or surface routes to move past the structure. Unfortunately, during the late fall and early winter, when overshoot steelhead are trying to return to their home waters, most of the dams in the system are no longer spilling enough water for adult steelhead to successfully pass along surface routes. Instead, these fish are forced to travel through the dams’ turbines.

Studies have shown that nearly 10% of the adult steelhead passing through a dam’s turbines are killed. If overshoot steelhead need to pass through multiple dams during their journey back to their spawning streams, then these losses compound. Mid-Columbia steelhead that overshoot high into the Snake Basin could easily end up crossing four or five additional dams, with each dam taking a toll on populations as the fish fall back downstream.

It is also worth remembering that steelhead, unlike salmon, can survive to spawn multiple times. In fact, these repeat spawners represent a fundamental aspect of life history diversity and resiliency among steelhead populations because of their high reproductive productivity. Unfortunately, kelts (i.e. post-spawn steelhead) migrating back to the Pacific suffer the same losses as overshoot fish when flows over the dams aren’t adequate, further contributing to population declines.

Photo: Ben Herndon

A Threat to Recovering Steelhead Populations

More research is needed to illustrate the specific challenges presented by the Columbia Basin’s overshoot steelhead, but enough is now well understood to see that these fish must be better accounted for in recovery and fishery planning and operation of the basin’s hydropower dams.

Large numbers of overshoot steelhead temporarily migrating into the upper portions of the Columbia and Snake watersheds means that managers are likely overestimating the actual numbers of fish returning to those tributaries.

When overshoot steelhead end up straying and spawning in tributaries other than their natal waters, the problem is a double-edged sword: Not only are these fish not contributing to sustaining the population of their home stream, they are likely spawning in places where their offspring aren’t adapted to survive as well.

Overshoot steelhead killed at the dams represent a devastating, frustrating loss and are undoubtedly slowing down the recovery, and ongoing viability, of populations that would otherwise be doing better. One of the most bitter lessons offered by tracked overshoot steelhead is that the basin’s dams are not only negatively impacting steelhead populations in tributaries above the dams, but they are also killing steelhead from many tributaries downstream of the dams. And when the fish cross multiple dams, the mortalities only grow.

By slowing the flow of the river and giving it more time to roast in the summer sun, the impoundments behind the dams are a leading cause of hot water temperatures in the basin, pushing steelhead to seek cold water refuges. Climate change will only increase water temperatures further, causing more steelhead to overshoot.

The numbers of overshoot steelhead in the Snake River are particularly troubling and are another reason why TU is working so hard to breach the four dams on the Lower Snake River and replace the services they provide. While it has long been understood that these dams are harming steelhead populations in rivers like the Clearwater, Grand Ronde, and Salmon, it is now clear that they are also harming steelhead populations in rivers like the John Day, Yakima, and Umatilla.

Learn more about TU’s campaign for a free-flowing lower Snake River and how to support this important effort at: 

If you are interested in learning more about overshoot steelhead and steps managers could take to reduce negative impacts from the hydropower system, be sure to read Gary Marston’s two-part analysis on the Wild Steelheader’s United blog. Find Part One and Part Two here.