Strainers, Large Woody Debris, Removal, and Ecology
How Much Wood Does a Paddler Chuck?By Kevin Colburn
Originally published in the American Whitewater Journal 2001.
We are not the first to pull logs from rivers. If you had been a paddler 300 years ago it would have been a very different experience. There would have been more beaver dams than you can imagine, and large numbers of logs scattered all throughout the creeks and rivers. It would have been a royal pain in the butt. Then the trappers came and killed all the beaver and on the heels of the trappers were loggers. Loggers cut all the trees that would have one day fallen into the river and built splash dams in steep creeks. These dams would hold back floating logs and then would be dynamited to flush the logs downstream. Logs were removed from large rivers to aid in passage of steam-boats, which were fueled with wood cut from the banks of the rivers. Later the army corps of engineers stepped in to do a really adept job of log removal in our large rivers to maintain their navigability. Farms, roads, railroads, and towns were built in the floodplains and along rivers. These structures required stable rivers that were free of logs and high flows. Many rivers were dammed, dredged, ditched, dyked, diverted, and dewatered to create straight simple manicured waterways. The policies of removing logs from streams persisted into the 1980s when it was still mandated that loggers remove all the logs from streams as part of cleaning up their mess.
Three hundred years ago most US rivers would have been rich with fish and salamanders and other critters of astounding diversity and numbers. The banks would have been densely vegetated in almost every part of the country, and where there were trees, there would be trees in streams. The rivers and streams would flood, but the stream and floodplain ecosystems were adapted to flooding. Salmon and other fish would ride the floods to the ocean, nutrient rich sediments would be deposited on the floodplains and the vegetation would flourish. There would be a constant cycle of natural disturbance that would create a river system that offered a large diversity of habitats for all kinds of species of plants and animals.
The European fear and hatred of things wild put an end to this amazingly rich and strong balance of natural forces and many river systems fell apart. Of the many damaging things our ancestors did to rivers, the worst was probably the removal of riparian (streamside) vegetation through logging, grazing, and development. A related impact that may be equally important was the removal of wood from streams.
It turns out that logs have incredible ecologically importance in many river systems. Logs trap sediment that would otherwise be washed downstream. They are a food source to aquatic insects and trap leaves and other important nutrient sources like salmon carcasses. Logs create pools and overhead cover that act as critical fish habitat. Logs dissipate stream energy that would otherwise potentially cause erosion to the bed and banks of the stream. Logs can create and maintain islands and gravel bars that increase habitat complexity. When logs are removed streams tend to become simpler. They will cut off meanders, have long uniform riffles, begin to down-cut and erode laterally, and will lose much of their biodiversity.
Obviously in most whitewater rivers complexity and erosion are not big issues because of the large boulders and bedrock that form the relatively stable bed, banks, and opportunities for fun. This is because whitewater rivers are typically steep and therefore have what ecologists call a high transport capacity. This simply means that they efficiently carry sediment and logs and anything else (us) through the area. When our rivers flatten out the water slows and sediment drops out of the water. Logs too are deposited on these floodplains. Logs typically enter whitewater rivers through random events like windthrow, mortality and decay, landslides, and avalanches. They will typically be removed by random events like floods, but may actually stay in the stream reach for long periods of time.
The dynamics of logs in slightly flatter streams are very different and less random. Floodplains are fairly flat areas adjacent to a stream or river. These areas can be tiny little groves in mountain streams or massive expanses along big rivers. If you are on a stream and see that the banks are made of fine materials like sand, gravel, or cobbles, and the land next to the channel is flat you are probably looking at a floodplain. Floodplain rivers often meander and have a fairly large amount of natural erosion and deposition. Logs most often enter these systems when the river undercuts the banks and causes trees to fall. Logs also can wash down from upstream reaches during high water. Logs in these areas are incredibly important to the stream and its inhabitants.
There is also a big difference in the role of logs in big rivers compared to small creeks. Logs generally stay where they fall in very small streams that are unable to move big logs. In medium sized streams logs are generally found individually and in jams that can be full spanning. In large rivers logs are generally found in large accumulations on islands and on the inside of meanders. In small streams one log can be very important, but in bigger rivers it is usually the accumulations that are critical.
So what does all this scientific knowledge tell us paddlers? Hopefully it will influence our actions on the streams where we paddle. As regular users of these creeks and rivers we have an important stewardship role. The science tells us that our sport may have evolved under unnaturally safe and easy conditions. It tells us that as our rivers recover from past abuse we are likely to see an increasing number of logs in our rivers. River restoration efforts are likely to become more common. These programs will likely include the artificial placement of wood in rivers. It tells us, perhaps most importantly, that we should seriously consider the negative ecological impacts of removing wood. If you are considering removing a strainer from a floodplain stream you will likely damage the stream. Removal of logs from steeper rivers with bedrock or boulder channels will have much less of a negative effect, but that log may have one day washed downstream where it is needed or be serving an important ecological function in that mountain stream.
If we as boaters cherish and respect these rivers then we have an ethical obligation to consider and lessen our impacts on them. When we are looking at strainers with pragmatism and disdain we should weigh the advantages that that log provides the stream with our own situation. In short, the stream's health should be considered before our whimsical urge to spend a few seconds paddling a few feet of river. Surely there is a set of continuums of ecological importance and another set that defines the log relative to paddlers. Some combinations of factors tell us we should not remove the log, others tell is it is more okay. Here is a shot at outlining these continuums in a sort of key. Think about these things if you are considering removing a strainer.
|Do Not Remove Log||Û||More OK to Remove Log|
|Sand, Gravel, Cobble Banks||Û||Bedrock Banks|
|Floodplain Adjacent to Channel||Û||Cliffs Adjacent to Channel|
|Log Trapping Sediment||Û||Log Above Water Level|
|Log is Large and Long||Û||Log is Small and Short|
|Stream has Endangered Species||Û||No Endangered Species|
|No Riparian Vegetation||Û||Dense Riparian Vegetation|
|Heavily Impacted Watershed||Û||Intact Forested Watershed|
|Log is Obvious||Û||Log is Hidden|
|Log is Avoidable While Paddling||Û||Log is Unavoidable|
|Log is Easily Portaged||Û||Log is Impossible to Portage|
|Log Unlikely to Entrap Paddler||Û||Log Likely to Entrap Paddler|
|Log in Seldom Paddled Reach||Û||Log in Popular Reach|
|Class V||Û||Class II/III|
Most paddlers have a strong environmental ethic and respect and even love rivers. We pride ourselves on approaching Nature on Nature's terms, not Man's. If we wish to paddle on Man's terms perhaps we can negotiate with Disney World, Dollywood, and 6 Flags to open their whitewater canyons to paddlers. However, if we wish to paddle on Nature's terms we will have to negotiate with Nature, in our hearts and in Nature's whitewater canyons. Lets come to that table well informed with ethics and caring and consideration. Lets leave the audacity so inherent in humans behind and work out an agreement that leads to rivers functioning up to their potential and paddlers safely exploring theirs.