Why maintain riparian buffers

stream shaded by forest riparian buffer
Forested riparian buffers help cool the water temperature in streams. Photo Andy Perleberg

Over Thanksgiving, I had the privilege to visit with family. There was a lot to be thankful for, not the least of which is my mother’s renewed health after a recent illness, and the bright eyes of our next generation shining from my young cousins. It turns out, all the things for which we are really thankful are those that allow us to all move forward together as a family and as a community.

In returning from a holiday to my professional community, these thoughts stay in my mind and I am filled with pride because we, as a community of foresters, move forward together. We have overcome so many technical challenges. Perhaps the most significant challenge we faced — protecting water quality — is resolved by maintaining forested riparian buffers. These buffers tie together the forestry community with everybody else in Washington State before any timber even reaches a mill. This practice protects water quality and salmon productivity while allowing our uplands to provide timber. These basic resources are all essential to a healthy, prosperous community.

But, how do forested buffers protect water quality and salmon populations? Well, it’s actually pretty simple. We tend to describe the ways that riparian forests provide clean water and productive fish habitat in terms of the ecological functions of these streamside forests, which are the mechanical ways that the forest interacts with streams.

Forested buffers intercept the loose soils that erode, or flow down the hillside when it rains and provide root strength in the slopes nearest streams to keep streambanks from eroding. Road use and timber harvesting often loosen soils in the uplands. Eroded soils, which are primarily mud, can fill in all the spaces between the cobbles, gravels, and larger rocks in a streambed. These are the only spaces where salmon eggs can incubate. When large amounts of mud flow into a stream, it’s possible to smother every salmon egg in the neighborhood, even downstream on other properties. Mud also prevents adult salmon from reproducing. Soils loosened from timber harvest activities can also erode slowly, leaving continuous source of soil particles in the water that must be cleaned before it is suitable for people to drink.

By maintaining shade over streams, we maintain water temperature. Cool streams are more important than they sound because high water temperatures stress fish, preventing adult salmon from surviving their challenging upriver journey and decreasing the survival rates of juvenile salmon. Warm streams also incubate bacteria and diseases that again must be treated before it is drinkable.

By providing litterfall and nutrients to streams, riparian forests supply the basic building blocks of a food web for salmon, especially for young salmon whose initial growth and survival depends on having great food in the form of small insects that decompose the litterfall and other raw nutrients. This food web extends into the Puget Sound and the Pacific Ocean where the salmon travel, feeding orca whales and countless other species. The food web that starts with riparian forests also feeds land-based species, like bears and people.

By providing large woody debris, riparian forests create the physical structure of instream habitat. The gravels, cobbles, and large rocks that I described above as being essential to salmon reproduction would all move downstream if the streams were straight, steep chutes. Instead, streams in the Pacific Northwest tend to be stepped with intermittent pools and riffles, resulting from changes in the slope of the streambed. This habitat is a result of wood that fell into the stream and blocked the downstream movement of gravels. As gravels build up behind a log in a stream, the slope of the streambed becomes more gradual. The pattern of small calm pools dotted along a stream with clean flowing water allows for the diverse habitat needs of fish. Salmon use pools to rest and feed. The riffles of slightly faster flowing water downstream of the logs provide well-oxygenated waters excellent for reproduction.

I am constantly amazed that forests provide such important resources like sustainable food and clean water. It’s an expensive practice to retain riparian forests during a timber harvest, but we cannot live without food and water. I’m thankful that we, as a community of foresters, have committed to responsible forest management. I’m thankful that we have the best information to protect public resources. I’m thankful for clean water and healthy food. I hope you take pride in every riparian area you protect because everybody in Washington, whether they know it or not, is thankful for clean water and healthy food.

Marty Acker, ESA Ecologist
NOAA, National Marine Fisheries, Washington State Habitat Office


The smell of home

salmon migrating
Fish, like this salmon, rely on a sense of smell to find their way ‘home’ to reproduce — if it means traveling trying to cross misplaced roads and other obstacles.

Fish have a sense of smell 500 to 800 times more acute than humans, and some species use that “super-power” to guide them to the right stream when it is time for spawning. What is it that they smell in the water?

Somewhere in the brain, smell and memory have a close connection. A whiff of a particular something can take any of us back to childhood for a brief second. But imagine a compulsion to follow that smell for days, always moving towards it in ever-increasing concentrations…back to home.

A trout’s sense of smell is estimated to be 500 to 800 times more acute than that of a human. Fish “smell” by drawing in water through a front opening or nare. The water passes through the nasal sac inside the snout where the olfactory gland can detect odors in concentrations of just a few parts per million.

One function of this amazing sensitivity appears to be to lead the fish home when it comes time to reproduce. Adfluvial populations of trout spend most of their life in rivers or lakes, moving to streams only to mate and spawn. Soon after fish hatch from the eggs, the smell of their natal creek is imprinted on the young fry. That particular home smell remains as a memory that lasts for as many years as it takes to reach sexual maturity.

Fisheries biologists can manipulate this imprinting to help reintroduce populations into a particular stream. In the 1990’s, hatchery-raised Kokanee fry were imprinted at the crucial moment with a synthetic chemical before being released into Lake Roosevelt to grow to maturity. Several years later during spawning time, the same chemical was dripped in the target creek to attract those adults to their new home. More commonly, biologists use in-stream incubators to imprint the desired smell (and location) onto the fish’s memory.

While research is ongoing as to what particular set of soluble chemicals evoke “home” for a fish, the most common theory is that it is the rocks, the vegetation, and other aquatic organisms that create the scent to which the fish return. Each stream’s odor is unique because each watershed that feeds it is unique. Imagine a row of glasses, each filled with water from a different tributary to your local river. Our native salmonid species could identify the source of each glass blindfolded, and perhaps tell us a bit about how our watershed management is affecting the flavor and smell. Fish notice when fertilizers and weed-killers are washed into creeks, or motor oil, or chemical de-icers, or (ugh) malfunctioning septic systems. With our weak human sense of smell, we can only imagine the degree of sensory input a fish receives from its surroundings. But imagining it may encourage us to work more diligently to “leave no trace” while recreating, to plant (or protect) streamside vegetation, and to restore functioning wetland areas. We may not smell the results ourselves, but for a fish… the nose knows.

Carol Mack
WSU/Pend Oreille County Extension

Why fish need forests and why forests need fish

logs in stream help fish habitat
Logs and other large woody debris help regulating the movement of material through streams and provides a variety of habitats for the fish and other wildlife that live and along the stream.

Why fish need forests

Stream habitat can be evaluated as a product of the various input factors that come in from the surrounding watershed. These can be divided up in various ways, and one way is to segregate them as geologic input of sediments to the stream, hydrologic delivery of water, and biological or chemical delivery of wood and nutrients to the stream. It is these characters together that define instream habitat and determine fish productivity, salmon productivity, and biological characteristics within the stream. Habitat and fish are the product of these inputs from the surrounding watershed, and it is through this approach that we can evaluate how forests influence fish.

The place where these interactions occur most is in the riparian zone, forests next to the stream. Three major interactions include:

  • Input of litter
  • Input of shade
  • Input of large woody debris

These are among the most significant when the issue of regulations is discussed. Input of litter is the delivery of the finer (smaller) types of organic matter from the forest to the stream: needles and leaves. This material serves as an important food source for invertebrates in the stream. For example, when one compares the inputs from early and late succession forests, about 60 grams per square meter per year of input occurred in the young (7 yr old) forest compared to 300 in the older forest. Some of the reduction in the young forest was compensated for by plant (algal) production in the stream channel of the young forest. Total organic matter availability in the stream of the older forest was about 80 percent greater. Forests do deliver a significant amount of organic matter to streams.

Shade and the role it plays in thermal regulation of streams is probably the riparian function that most people are familiar with. Studies have shown that presence of forests keeps the average temperature lower. Forest cover also keeps the day-to-day and day-to-night fluctuations lower as well during warm periods of the year.

Input of large wood to streams has become a very hot topic in scientific and management circles over the last 10 to 20 years. Wood has a variety of effects on stream systems: 

  • Impacts on channel form
  • Physical characteristics of stream channels
  • Wood plays a key role in regulating movement of material through stream systems
  • It provides a variety of habitats for stream biota

Wood has a dramatic effect on the shape of the channel, helping to form pools. In small to moderate streams 60-80 percent of the pools can be related to coarse woody debris. Local scour around the piece of wood creates a pool with deeper water. For a long time, people thought that large woody debris did not play much of a role in determining channel form in large river systems, but in recent years we have found that wood does play a significant role in channel form in these large streams. Pieces that play such a role are invariably large trees with intact root wads, introduced into the channel from bank erosion. These trees get deposited fairly close to the channel bank, with the root wad oriented upstream and the rest of the tree trailing downstream. It begins to trap more coarse woody debris on its upstream side that creates slow flow on the downstream side and creates sites for sediment deposition and vegetation establishment. Wood influences channel form and successional dynamics in the riparian zone.

Sediment is retained by large woody debris and influences its movement through the stream system. The transport of fine and coarse sediment increases twofold in the absence of wood. Increase efficiency of transport, and mobilization of stored sediment, will both occur. Wood can also retain organic material in the stream longer than if wood is not present to trap smaller material. Not only needles and leaves, but also salmon carcasses can be retained by wood in the channel. Up to 60 percent of salmon carcasses can be associated with presence of coarse woody debris.

Wood plays a key role in providing habitat for animals. Spawning adult salmon will typically will use large pools, many of which are associated with woody debris, for holding areas prior to spawning. Many of the fish species of high interest will use similar pools while juveniles are rearing in fresh water. Comparison of cutthroat trout and coho salmon biomass (abundance X body weight) in British Columbia showed far higher biomass in sites with high amounts of large woody debris.

The entire watershed interacts with the stream channel. The delivery of water to stream channels will change, for example, with development of a forested watershed. As a result, peak flows will increase. In King County the average flow expected once every five years in a forested watershed was occurring once every year in an urbanized watershed. Biological consequences of this are increased bed scour, and reduced fish egg survival. Emerging fish are more likely to be flushed downstream, and food availability may also be less. Productivity of streams for coho salmon declines rapidly, so that in streams in King County with 10 percent impervious surfaces, salmon productivity had declined 70 percent from forested watersheds. At 40 percent to 50 percent impervious surface, these streams are essentially inconsequential in terms of salmon production.

Why forests need fish

The reason forests in the Pacific Northwest need fish is because of the kinds of fish we have here. Pacific salmon exhibit two characteristics that enable them to impact the habitats and forests of the streams they live in:

  • They return to spawn in the same place, and in large numbers
  • They die after they spawn

 So they transport marine-derived nutrients to generally nutrient-poor headwater areas where they were born, and leave them there by dying. The kinds of effects salmon have on freshwater habitats include:

  • Alteration in channel form as a result of the physical act of constructing redds, and this is particularly true for species that spawn in high densities, like chum and pink salmon.
  • Changes in the characteristics of gravels, by removing large quantities of fine sediments.
  • They are a food resource for wildlife as well as other fish and invertebrates in the stream.
  • They are a significant nutrient source for riparian forests, too.

About 20-40 percent of the nitrogen and carbon in the body tissues of cutthroat trout, coho salmon, and steel-head are derived from marine sources. About 18 percent of the nitrogen in the tree foliage is also of marine origin, directly attributable to salmon. Subsequent work has shown that leaves of salmonberry, a riparian-oriented shrub, that nitrogen content is higher in streams that contain salmon than in streams without salmon. Increased understory cover and species diversity has been associated with streams where salmon spawn, and tree growth may also be increased significantly. This, of course, has a positive feedback in that potential pieces of large woody debris are being developed faster and fall into the stream to improve habitat quality.

We have been trying to understand fish utilization as a function of the primary land use in a watershed. The Snohomish River watershed shows that median coho salmon abundance declines significantly from forested watersheds to rural residential use to agriculture and urban areas. The only locations with consistent high utilization by salmon are the forested watersheds.

Salmon recovery should pay special attention to retaining lands in forest cover. Because of the way we currently manage these other lands, they tend to be very incapable of supporting high populations of salmon.

 Robert Bilby, Ph.D.