Skywalker-

Actually it's a little of everything you mentioned, but tends to depend on the type of slide and the amount of damage you want to correct. There were many attempts in the early 80's to develop a machine that would remove sediment from gravel, but the feasiblity (financial and technical) just wasn't and likely isn't there. Some of you may even remember the UofW's gravel gertie that was developed to clean spawning gravel. There's also the issue of the cure sometimes being worse than the disease.

On a large slide that dumps thousands of yards of dirt and sediment into a river, you'd have to build haul roads in riparian areas just to move the sediment exctracted from the stream. In the process, you'd loose riparian cover, create fish passage issues when crossing tributaries and a variety of other issues. You'd also have to imagine this huge machine chugging down the river disturbing the streambed and any fish present. Not a pretty picture in my mind.

Just for the record, I've seen a couple of big slides that I actually thougth improved the stream dramatically. I know this sounds counterintuitive, but a stream that lacks large woody debris and gravel can actually benefit from a slide or debris torrent. I know in one instance that within a couple of years of the slide I found a lot of new pools and riffles that provided excellent fish rearing and spawning habitat that previously didn't exist.

As for the slide itself, that's another ball game. Slides that strip all the soil and leave nothing but rock are extremely difficult to restore. The only way to really restore the slide area is to import soil and plant vegetation. Then there's the issue if the soil can be made to "stick" to the rock and not runoff in the first big rainfall. There's also the problem of accessing the site to get the soil to the slide. On steep slopes your only real option is to use a helicopter and moving thousands of yards via helicopter would bankrupt most landowners.

In a situation like this, if the sky was the limit I'm sure we could develop some sort of mitigation, but I'm not sure what it would be. On slides that don't strip all of the soil it seems best to revegetate the slope and identify the trigger mechinism so we can make sure it doesn't happen again. Sometime this means removing or improving roads or improving drainage features.

There's a lot to evaluate when it comes to slides and I could spend days writing about it, but hopefully this sums things up enough to answer your questions.