Science of the seasons: Stream debris can be good for fish, insects

The first raft or boat trip on the Kenai River each spring is a new adventure because the river will have changed significantly since the last trip in the fall. There will be new channels and unknown shallows that have to be noted in order to avoid costly repairs to the boat bottom or prop. Along with the substrate changes are the arrival or removal of large logs and, sometimes, entire trees. Sections of a river with significant numbers of logs and stumps are usually given a wide berth by boaters.

Logs and stumps are transients in the river until a large flood or ice jam washes them downstream. Eventually they will end up in the inlet and can create their own hazard out there. While in the river, however, the logs can have a significant impact on surrounding substrates and can become a temporary microcosm of riverine organisms, especially fish.

Most floating trees and logs are the result of normal bank erosion as the river meanders within its flood plain. “Sweepers” are frequently seen on the outside of river bends as the tree-supporting substrate is slowly washed away and trees bend over the river. As more and more of their support washes away, they dangerously skim the river surface until being completely uprooted. Once floating free, the strong current carries them downstream to the first shallow section. The limbs and branches are lighter and have more surface area than the stump and trunk end. Because of this, the heavier stump end tends to get dragged, begrudgingly, downstream.

Once a log or tree becomes “grounded” in the river, it starts changing the river substrates nearby. Water washing up against the unmoving trunk or stump will create a deeper hole below or to the side. Other areas along the log will slow the current so that sand, gravel and cobbles will be deposited there. By deflecting parts of the current, new channels will be created around the log. Some of the substrate and channel changes will in turn cause the log to be washed farther downstream. The process begins again as soon as the tree stops.

Once one log gets solidly lodged in the current, others seem to be attracted. In a fairly short period of time, with the arrival of new trees, large logjams can develop. As the numbers of logs build, so does the impact on the original water-flow pattern. Logjams can cause water to be diverted far from the original course and new side channels can be formed. These side channels create a variety of new pathways for the water to flow downstream.

Streams with numerous side channels are much more difficult to navigate with a motorboat than those with a clearly defined channel. However, rivers with more logjams, as well as an increased number of side channels, seem to have much fewer flash floods compared with rivers that have had logjams removed. By diverting water into numerous areas of the flood plain, the water moves downstream more slowly and is less able to erode away constraining lateral banks.

Various aquatic insects will use trees and stumps for an in-stream residence. Since many insects feed on fine, drifting organic particles found in the water , a spot on a tree limb with water drifting past is the best seat in the house. Some, like hydrospychid caddisfies, build filtering nets on these wooden substrates. Others, like black fly larvae, attach their abdomen to the limbs and filter out fine particles with special antennal fans. Because algae and diatoms will grow on the submerged wooden substrates, algal grazers will also be attracted to the logjam. In streams with soft, muddy bottoms, logjams and submerged trees can be some of the only available insect attachment sites.

For fish, and the astute fisherman, logs and stumps along a river are attractive areas. Because of the deeper channels around the logs, fish can find passageways if they are moving upstream. There are usually sections underneath the logjam where the current is slower than the surrounding stream and these areas are used as a fish refuge. Other fish will remain in the deeper sections underneath the log and wait for drifting or dislodged insects to wash right to them. Light shadows created by the tree or its limbs will also camouflage fish from potential predators. It’s kind of like an all-you-can-eat buffet for the fish in a sheltered restaurant.

Like virtually every other part of a stream or river, there are constant changes in the submerged trees and logs, as well as the rocky substrates around them. While these dynamic habitats can be dangerous for boaters, they can be a blessing for aquatic insects, fish and fishermen.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River watershed.