Off The Beaten Path – The Index Eagle December 1990 by Bob Hubbard

This article is from the Index Eagle, December 1990 and authored by Bob Hubbard. He has given us permission to reprint his articles but PLEASE DO NOT PLAGIARIZE. This article may not be reproduced without the express written permission of IndexWa.org and/or Bob Hubbard.
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The floods this year have done more than just create a lot of environmental havoc and property damage – they have also rekindled many peoples’ interest in forestry as it relates to watershed dynamics and flooding. Contrary to what a lot of people seem to think, these floods cannot be blamed exclusively on the loggers and their handiworks, for there are many, many factors which operate together to create a flood. Because of the speed with which the river rose after the storm started. I suspect the influence of a least one of these factors, though – the one I call the Damaged Sponge Effect.

The uncut forest presents three layers of sponge to rainstorms, each of which soaks up rain until it is saturated before it passes the rain along, undiminished to the layer beneath it. Logging can destroy or damage each layer, reducing the water holding capacity of the site and also the amount of rime it takes to saturate the site, which then starts shunting water from the storm into the creeks. This means that more of each storm winds up in the creeks, faster, in the damaged sponge areas, than from undamaged areas. The tree crowns make up the first layer. Old-growth trees, possessing immense quantities of needles, branchlets, lichens and mosses often take a half hour or more to become saturated (as anybody who has ducked beneath a giant tree to get out of a summer squall knows). After leaf-fall, this layer is less effective in hardwood forests, such as our pervasive Alder forests of the Skykomish; and in clear-cuts this layer is completely gone.

The second layer is the litter/duff layer of the forest floor. In old-growth, this layer is especially voluminous, having been accumulated over hundreds of years. Thick moss and well-rotted logs also contribute to the effectiveness of this layer in delaying the arrival of rain to the next layer down. The widespread practice of slash-burning partially or completely destroys this layer, as does the skidding of logs and the building of roads.

The third layer is the soil itself. Under old-growth, this layer is typically deep, with lots of pores, and has tremendous water-holding capacity. Unfortunately, the pore structure is quite fragile, and the thump of a falling tree, or even a few passes of harvesting equipment is sufficient to compact the soil enough to largely destroy its water-holding capacity. Slash burns can destroy the crumbly structure of the soil, giving the same results, and the bared soil is exposed to direct rain splash (rain often falls with sufficient force to blast soil particles a meter into the air). Sediments loosened this way are washed into otherwise undamaged soil pores, plugging them. This layer is very slow to heal, following damage.

As can be seen, there are many ways that logging can damage the sponge function of the forest, thereby hurrying storms into the streams and causing fast rising rivers during rain events. The more a watershed is logged, the more potentially damaging this effect becomes. Although modern harvest techniques are less damaging to the sponge layers than some of the older techniques, the fact remains that significant percentages of our watershed were clear-cut and burnt.

In spite of years of new growth on the soils of these sites, the sponge function remains severely impaired on them, so we’d better get used to rivers which react fast and extreme to every passing storm.