Imagine how it all started...
A river started flowing. Salmon started returning. Trees started growing. It rained, the riparian zone absorbes the water, purifying it and slowly releasing it into the river acting like a resorvoir in the true sense of the word. The river raises, the Salmon die and are left on the bank to fertilize the Riparian zone and feed the various animals which inhabit the forest. Wind storms happen, or trees get too old, and they fall down. They fall into the river where the next flood carries them down river to a sand bar where they get lodged into the side of the rip rap. Here they stabilize and protect a bank from erosion, and scour the bottom of the river which creates important cover and pool habitat for Steelhead Juveniles. More floods occurs, and more Large Woody debris is washed down into the riverbed creating more and more habitat. The Large Woody debris catches Salmon carcasses and holds them there until the next Spring where they will fertilize the surrounding river and jumpstart the food chain.
Now if you put two and two together you can see where a logged watershed fails. It raises and falls very quickly. It has very extreme floods, and since the riparian zone doesn't hold back water, it washes everything into the river. Dirt, sediment, fine gravel. There will be some Large Woody debris, but when the extreme flood settles the debris will be high up on the bank where it serves little use to that of the stream rearing fishes. It won't catch carcasses and it won't create any habitat.
Like Ryan said, the fine sediment covers the river bottom. Here it will cover the rocks which destroys the ability of algae and any nutrients in the stream to fertilize the algae on the rocks denying them the completition of photosynthesis. Without algae, there is no bug life, for the invertebrates use algae as a food source. Eventually the fish which use the stream are left starving because the streams ecosystem has been destroyed.
I think you get the point.
On a side note, you have to watch things like clay banks. There are a few clay banks up here too believe it or not (Yes, we have clay banks in Canada!). What can happen, since many clay banks are on a steep face, is the river will erode away at a clay bank and leave an overhanging piece of clay creating an undercut bank. Eventually with some rain, a slide will occur and it could put the river out for weeks and this whole undercut will fall into the river.
From my understanding, Deer Creek is where Roderick Haig-Brown caught his very first Steelhead.