Science Friday: How do steelhead survive in intermittent streams?

It’s the first Friday in October — officially, “Septober” for us steelheaders.

 

Unfortunately, steelhead runs across much of the West Coast have been down this year, and now there is another blob of warm water in the North Pacific. Things don’t look good right now, but wild steelhead have withstood these types of conditions before. As long as we continue to protect their habitat and manage the fish based on the best science, wild steelhead have a chance to persist and even thrive.

 

In this post we will look at one of the most basic requirements for us to have wild steelhead in our future: clean water.

 

We tend to take clean water for granted nowadays. After all, the federal Clean Water Act, enacted in 1972, has been one of the most effective environmental laws in U.S. history. Rivers in America no longer spontaneously catch fire due to high pollution levels (as the Cuyahoga River did exactly fifty years ago).

 

But we shouldn’t take good water quality for granted. Not when the current administration, through the U.S. Environmental Protection Agency (EPA), is trying to eliminate protections for certain types of streams under the Clean Water Act.

 

In particular, the EPA is proposing to eliminate Clean Water Act protections for streams that do not have continuous surface flow — so-called intermittent streams. Intermittent streams lose surface flow for part of the year, typically during summer. These are often smaller creeks, such as those found in the headwaters of watersheds, but they are almost always tributaries to larger water bodies.

 

While such streams may not seem that significant, in reality they provide very important habitat for many species of salmon and steelhead. And pollution always flows downstream. If we truly want fishable populations of steelhead, we need to ensure that we are adequately protecting all the types of habitat they rely on.

 

This week we review a recent study by three scientists at UC Berkeley that investigated the factors influencing the survival of juvenile steelhead in in two small tributaries to Salmon Creek, located north of San Francisco (Study can be found here: https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecs2.1645). Each year some portion of both of these creeks goes dry, and in the worst years there are only relatively short segments of each stream that maintain surface flow.

 

 

However, one of the two streams had a higher frequency and greater duration of intermittent flows than the other. The more intermittent stream channel was predominated by gravel deposits for its entire length, while the other creek’s upper section was more confined by clay and bedrock canyon, where flows were rarely intermittent.

 

Other studies have found that intermittent streams tend to be predominated by gravel deposits, which are porous and allow surface flow to seep down into the aquifer. In such cases, it is common to have shallow riffles that eventually dewater into stranded pools where the stream has scoured the gravel to a depth that reaches the underlying water level.

 

The authors measured the size, shape and depth of such pools and monitored water temperature, dissolved oxygen, and the survival of juvenile steelhead (and coho salmon) to determine, 1) survival from early-summer (June-July) to fall (September-October), and 2) the factors that influenced survival.

 

Which stream had higher survival of juvenile steelhead?

 

(Before we answer that question, it’s important to note that both streams support a relatively high level of juvenile steelhead and coho salmon, but as with any study, the survival and abundance varied across years.)

 

This study found that, overall, the stream that more frequently went subsurface had higher recruitment – measured as survival of juveniles from early- to late-summer – except for one year, when extreme drought conditions resulted in nearly equally poor survival for juvenile steelhead in both streams.

 

While this result may seem counterintuitive, research on juvenile coho salmon on the Oregon coast found that mortality can be pretty high in stranded pools, but that those remaining survivors may grow well enough to pass up cohorts that lived in streams that did not go intermittent.

 

In other words, don’t write off the creek because it’s stingy with its surface flow.

 

In fact, it’s amazing what the fish endured and that they survived at all. In this study the juvenile steelhead, and coho, persisted for weeks in pools with low levels of dissolved oxygen (DO), including pools where part of the water column reached sub-lethal or lethal levels. Consistent with the findings of other research, it is likely that the temperatures remained just cool enough and the DO levels just high enough because there was shallow groundwater flow moving subsurface through the gravel and into the pool.

 

Based on its thorough measurements and monitoring, this study’s modeling revealed that juvenile steelhead tended to survive best in pools with a larger surface area, cooler water temperatures, and with more juvenile steelhead. Steelhead were able to persist for only short periods of time in smaller pools, indicating that pool volume and depth are critical features in intermittent streams.

 

 

Survival of juvenile salmon and steelhead is often strongly correlated with flow in California, which shouldn’t be a surprise considering its drier climate and steep demand for already limited water supplies. Nonetheless, as we see in this study, steelhead can survive in intermittent streams and in fact, as the authors note in the discussion, the abundance of juvenile steelhead and salmon can be very high in such habitats.

 

Of course, in really dry years, when flows become even more restricted, fish don’t survive as well. But in drought as in wet years, protecting water quality in intermittent streams is a no-brainer for steelhead and salmon conservation.

 

Larger main-stem rivers and perennial tributaries remain, in general, the most critical habitat for spawning and rearing steelhead and salmon. But that is not always the case. Populations of steelhead in Southern California rely heavily on intermittent streams — as they do throughout Southern and Eastern Oregon, Idaho, Eastern Washington, and even along the coast from Northern California up and into the Olympic Peninsula. One of the most important winter steelhead spawning tributaries on the OP, for example, is intermittent and is smack-dab in the center of the rainforest. Each year the adults spawn and the juveniles emerge and eventually migrate downstream, sometimes several miles, to sections that have surface flow.

 

The remarkable diversity that is the hallmark of wild steelhead is not just because of their genetics. It’s also because they occupy a wide range of environments. A life history that solves a problem in the main-stem river likely differs from the life history that solves a problem in an intermittent stream. One fish doesn’t need to move early in life, another does; and the different environments will produce different growth rates.

 

Steelhead diversity is a reflection of the entire tapestry of habitats available to them. If we lose a component of their habitat, we lose that life history. Conserving wild steelhead depends on conserving their diversity, which requires providing healthy and diverse habitats. This study, and others, have clearly documented the importance of intermittent stream habitats for steelhead and salmon. If we really want to have fishable wild steelhead populations in our future, we need to step up now and make sure these habitats stay intact. That starts with ensuring the water that flows through them–even if seasonal–is clean and cold.