Large Woody Debris

Trees. Shoreline vegetation. What a nuisance they can be when we all are working moving water on the fly. How often have we lost that special fly? That fly that was working so well, lost on that shoreline tree! The last of the best fly in our box of the day. But this vegetation is vital to the health of our streams, as well as our fly fishing experience.

I am gaining a better appreciation of that riparian vegetation as I work the rivers and streams of the Pacific Northwest. Most of our streams have been modified to such a degree so as to no longer resemble the free-flowing streams that existed prior to the Hudsons Bay Company.

Prior to our settlement of this land, streams flowed freely, moving and shifting. Scientists call it channel migration. Streams owned our valley floors, shifting over time, usually over decades, claiming and returning longer tracts of the valley floors in the process of migrating.

Shoreline trees play a most important role in the process of natural channel migration. Large woody debris, specifically, are essential to stream migration and fisheries habitat.

I didn't fully understand the working of large woody debris until this past spring. During spring runoff on the Yakima, an enormous Ponderosa pine dropped from the shoreline edge to land square across the river. Most trees-usually cottonwood-fall in a more downstream direction. That is, the direction of the wind-almost always downstream. Not this pumpkin. It fell perfectly perpendicular to the shoreline, its main stem halfway across the channel. The pine's configuration presented a formidable obstacle to drift boats and rafts-and me in my waders.

What I didn't understand was why this particular pine? Its base was situated 30 feet from the normal high water, well away from the current or shoreline erosion. On the opposite bank stands another pine, whose roots were half exposed by a heavily eroded shoreline. Many times, I had observed this tree, fully expecting it to fall at any moment in the path of a strong wind gust. But no, it still stands.

The Pumpkin, on the other hand, fell while still firmly set on land. Or at least that's what it seemed. I took the time to follow the trunk of this great tree from stream to base. The tree was fully four feet in diameter at the base! What I discovered helped add to my understanding of the river.

The base of the tree was fully uprooted. The tree had not broken- it had tipped over! The pine's roots were wet and rotten. Certainly, there had been plenty of fine fibrous membrane to carry food and water to the canopy. The roots were so wet they had weakened over time.

What we are reminded by this phenomenon is that a river channel is quite complex. While the channel itself is well defined, the actual shoreline riparian zone can and often does extend hundreds of feet from the edge of the flowing channel. The dimension of this influence zone varies with the size of the stream and the geology of the basin. On the Yakima, this zone can be several hundred feet wide.

It is within this zone-the channel migration zone-that the river takes claim to large woody debris, so vital to the health of our streams and our fisheries. Most often, the claiming of large woody debris by the river is direct and dramatic-the result of the working of the river and the erosion of the stream banks. In the case of the Pumpkin, the claiming was subtle. Upon investigation, the tree fall was result of groundwater movement along a shallow shoreline.

The result of this particular reclaiming has been dramatic. The tree fell across a broad riffle, fully fifty yards in length, just above a small island.

After several weeks of high spring runoff, the riffle has changed dramatically. The tree has shifted to a downstream direction, yielding to the power of moving water. The riffle is now half its original size. On the downstream side of the tree, the riffle is non-existent. A large pool has been created and the riffle replaced. Above the tree, the riffle has also been eroded.

The fishing characteristics of this stretch of river have been changed significantly. This was a piece of water I knew well. It was one of my favorites.

The presence of such large woody debris in our special streams enhances the fishing experience. I will now have to learn the new holding pockets. I will watch new riffles created as the stream works its way around the pine depositing gravel and stones in new and different configurations. I will learn this new water over time. And when the next pumpkin falls; I'll learn it all over again.

Len Zickler