Stupid New Guy Question - Gravel Mining

[mic]

If good river water needs a mix of deep holes, then why is limited mining of river gravel down to the bed rock...thus making a shallow hole deep...a bad thing?

[Riverwhy]

It is not a stupid question at all but it is certainly an emotional one for many of our OA members. Removal of excess gravel from our stream beds can be a positive influence on the fishery. We all wish that the land clearing would have stopped long ago and that erosion into our streamas was less but the reality is that there is way too much gravel in the streams today. If the operation to remove the gravel was accomplished with minimal bank degradation and was done over a realitively short period of tiome the effects could be positive. However, continuous mining creates downstream sedimentation that could be very destructive.

Just my opinion.

[fishinwrench]

Right. The dredgers seldom if ever take into account any after effects of their gravel removal, they choose only to pull it out where it is most convenient for them to get to. When they remove gravel all the way to bedrock it begins tearing the banks down resulting in wide shallow areas. And where those wide shallow areas neck back down it REALLY eats the banks away during high water.

A heavily dredged stream will eventually become overly wide and overly shallow. If there's not enough deep water holes then the stream can't support a healthy population of gamefish.

When water moves over a wide shallow bedrock stretch it warms up drastically under the Summer sun making the area for several miles below like a hot steamy sauna. Fish don't like that.

[Wayne SW/MO]

I think they answered your question Mic, but I might add that gravel mining and dredging aren't one and the same.

[Al Agnew]

Let me add to what was said...

The farther away you get from the stream bed, the less damage is done by gravel mining, so the least destructive way, where possible, is to dig it out of the bottom fields away from the river...as long as you leave a buffer zone that is wide enough and well-vegetated, so the river doesn't decide to change its course into the hole you've dredged.

Second least destructive way of getting gravel is called bar skimming. This is where you scrape gravel off the bars, not digging down as deep as normal water level, and leaving a buffer zone between your skimming and the water's edge. By not digging down to normal water level or below, you don't encourage the river channel to move into your diggings. But there ARE problems with doing this, the main one being that it often destabilizes the bar. "Natural" gravel bars usually have some vegetation growing on them, and even if they don't, they have a veneer of somewhat cemented gravel on the surface. As you'll know if you've ever driven a vehicle onto a gravel bar, you can drive out and stay on top of it, but if you break through the surface at any point you'll immediately sink up to the axles. That veneer of stuck together gravel often but not always keeps the bar stable when you get high water that barely covers the bar, so the gravel doesn't move. But if you destabilize the bar by digging the top of it off, the gravel beneath will move like crazy in the next high water. Best example I can give of this is a big bar on the lower Huzzah. There used to be a deep hole about a quarter mile below that bar, and the pool never changed much, and neither did the bar, for a number of years. Then gravel miners moved in and bar skimmed that bar...and after the next high water event the pool below was filled with gravel and totally ruined.

The most destructive ways of removing gravel involve just what you suggested, digging it down to bedrock or otherwise digging deeply into the channel of the river. Most of the time, doing this requires removing a lot of vegetation along the banks, just so you can get your dredging stuff deployed, as well as to get gravel that is lying along the banks. So that's the first problem, removing the vegetation removes shade, widens the channel, and warms the river.

Second problem with this is that any digging within the channel might deepen the river at that point, but it removes all cover, including logs and rocks and weedbeds.

Third problem is that just mucking around in the channel stirs up enormous amounts of silt, which then get washed downstream to make long sections of the river murky to muddy, and to fill in the pools downstream with silt.

Fourth problem is one that is a little harder to envision but is a VERY big one. Picture this...you have a gradually sloping river channel, a little deeper here, a little shallower there, typical of riffle/pool streams, even those that are filled in with gravel. Now dig a big, deep hole in that gradual slope. The river "wants" to get back to the equilibrium of that gradual slope, it doesn't like abrupt drops and rises. The only way it can do that is to start digging deeper upstream to bring gravel from upstream down into the hole. What this means is that a formerly stable channel upstream suddenly destabilizes as the river begins to dig more and more deeply into its banks in order to get back to the gradual slope. Now you have eroding banks upstream, trees falling off the destabilized banks, and gravel moving everywhere. It even extends into any tributary that's nearby.

This is not theory. There have been many studies done on streams upstream and downstream of deeply dredged pools, and they all show increased bank erosion and increased movement of gravel, often of catastrophic proportions.

So while you might get a temporary deep pool if you dig it out, the pool quickly fills in, and the banks and gravel destabilize upstream, often for miles.

And if you dig out a long section of stream down to bedrock, you've lost that whole riffle/pool dynamic, and instead have a channel shaped only as much as the bedrock is shaped, which usually means it's wide, and uniformly shallow, not good.

The amount of gravel in any Ozark stream is of such massive proportions that ANY deeply dredged area fills in from upstream, often very quickly. I can take you and show you a whole bunch of gravel mining areas on a bunch of streams in the eastern Ozarks, and without exception, those areas are worse habitat than the un-mined areas well upstream and downstream. Some of them have not been touched for more than 40 years, and the damage is still noticeable if you know what you're looking for. And even if you could dig out long sections of a stream, there is always more gravel coming in from tributaries and ravines, and any benefit will be temporary at best, and more likely non-existent.

[mic]

That makes a lot of since. Thanks.