All of us who float, boat, and fish Ozark streams have noticed that our favorite waters seem to be getting shallower, with pools filling in and habitat degraded in some of the spots we fish. There are constant calls to do something about this, most commonly from people convinced that we need to allow more gravel mining. But is that true? Nope.
Gravel is a part of the geology of the Ozarks. The limestone and dolomite that make up much
of the surface rock strata is weakly soluble in rainwater, and the water
dissolves voids within and between the layers.
Chert forms in these spaces, and some formations also produce drusy
quartz, the same mineral as chert—silicon dioxide—but in masses of crystals
that form on the inside surfaces of the voids.
Silicon dioxide in either form is much harder than the matrix rock and
remains on the surface as the limestone is dissolved and eroded away. So all the streams have sources of gravel,
though some flow through formations that produce more gravel than others.
Gravel erodes off the hillsides with heavy rain, into the
hollows, creeks, and tributaries that feed the streams. This has been going on since at least the
last ice age, when glaciation occurred almost to the latitude of the Ozarks. At the time of the greatest ice cover, the
Ozarks was probably treeless except perhaps in small, sheltered areas, and when
the ice began to melt, the climate was wet and erosion was rampant off the
nearly bare hillsides. The evidence of
this is found in the thick layers of gravel beneath the topsoil in the
bottomlands of the larger streams, gravel that has also been mined for many
years, and which one can often see in the places where the stream is eroding a
bank; there will be a layer of alluvial mud, and beneath it, near the normal
water level, a layer of gravel. If we
think gravel is bad in the streams now, we should have seen it 10,000 years ago! The valley bottoms were probably almost all
gravel, with the streams running in braided channels through those extensive
gravel beds.
Thousands of years of weathering and forest growth changed
the streams and valley floors. As trees
and other plants covered the gravel bottoms, they trapped topsoil that built up
in thick layers over the gravel. The
forests became old growth; big, widely spaced trees with native grasses growing
beneath, maintained by natural and Native American set fires. The growth held the gravel on the
hillsides. Gravel already in the stream
channels was washed downstream and ground away until at the time of the first
European settlers, there were many sections of streams flowing over bedrock
bottoms, as reported by Schoolcraft and others.
But then came the great logging boom of the early
1900s. The forests were cleared. Brush grew and shaded out the grass. Burning to remove the brush and “kill ticks”
was common and widespread. The gravel
began a new cycle of erosion into the streams and tributaries. In old photos from the period of the 1920s to
1940s, Ozark hillsides are covered in sparse, scraggly trees, with brush and a
lot of bare ground, perfect for widespread erosion.
By the 1930s, the streams were probably in the worst shape
they had ever been during historic times; not only from wholesale erosion, but
from log drives and railroad tie rafts that destroyed banks, uprooted trees,
and scoured the bottom of the stream in shallow water. It took until the 1950s before the forests
had grown back enough to provide some protection from erosion, but the native
grasses that were best at stabilizing hillsides never returned, and gravel
continued to enter the streams. Logging,
housing developments, clearing of hillsides for pasture, roads, and ATV and
horse trails have proliferated over the watersheds, all contributing to more
gravel in the hollows and tributaries.
Most people would be surprised at the amount of gravel
continually coming into the rivers. I
saw this firsthand many years ago. There
had been a massive rainstorm that was centered over one small area of Jefferson
County through which Big River flowed. The
river got no rain upstream of this area, so the upper river didn’t rise. Instead, it stayed normal until reaching this
small area, where the massive rainstorm caused a rise beginning in this one
section. I floated from Mammoth Bridge
to Brown’s Ford a couple weeks after the flood that ensued. I could tell from the mudline on the
vegetation at Mammoth that the river had only risen a few inches. But the farther I went downstream, the higher
the mudline was, until at the end of the float, 10 miles downstream, it had
been 8 feet higher than normal.
Amazingly, at the mouth of every tiny creek and hollow there
was a delta of gravel and even rocks up to the size of a basketball, and some
of those deltas extended halfway across the river. There had obviously been a massive amount of
gravel coming down those wooded hollows into the river. But in a usual flood, where high water coming
from upstream would have scattered that gravel as it entered the already high
river to the point where it was unnoticeable once the river dropped, it hadn’t
gotten high enough upstream to move the gravel.
Crucially, huge rains like this are becoming more common
with the changing climate. It doesn’t
matter whether the change is human-induced.
The fact is that huge floods are occurring at a greater frequency than
they did throughout much of the 20th Century. They are often occurring almost back to back,
with two or more floods within a very short period of time. Examining the US Geological Survey records
for the Sullivan gauge on the middle Meramec, you’ll find that from 1915 to
1970 there were six floods that exceeded 40,000 cubic feet per second, only two
of which reached 60,000 cfs. From 1970
to 2025, the same number of years, there have been 12 such floods, including a
new record flood and six more floods over 60,000 cfs. So there is more erosion occurring, which
means more gravel in the streams.
But new gravel is not the sole reason, nor perhaps even the main reason, that
the rivers appear to be getting shallower.
River channels are widening.
The same volume of water will be deeper in a narrow channel
than in a wider one, and narrow channels also increase the force and power of
the water, making it better at scouring out pools and maintaining them at high
water flows.
So why are the streams widening their channels? It all starts with destabilized banks. It is difficult for many people to
understand, but while floods do the damage, stable stream banks lined with
trees and vegetation are usually not susceptible to erosion in a typical flood. The intertwined roots hold the bank together,
and the flood’s power cannot find a weak spot to attack. It takes a truly huge flood, or some kind of
damage or change in the bank, to make it vulnerable to erosion.
The most obvious cause of destabilized banks is landowners
clearing the trees to water’s edge, removing the protection those tree roots
furnish to the banks. In the 60 plus
years I’ve been fishing Big River and the Meramec, my two “home” streams, I’ve
seen the river erode more than a hundred feet of some of those cleared banks. In several places, I can show you where the erosion
has removed an entire bottom field and the river is now up against a rocky
hillside. And invariably, where a bank
like that is eroding, gravel is building up on the other side, which has become
the inside of a bend. You will see a
gravel bar, then a zone of young trees, with progressively bigger trees the
farther away you get from the river channel.
The river’s original bank with mature trees might be hundreds of feet
behind those zones of growth. Landowners
see the damage, but by then it’s very difficult to fix. The erosion must first be stopped, and that
usually means hardening the bank with riprap rock (or, in the past, anything a
landowner could get hold of, including old car bodies). Some landowners have attempted to use just
piles of big rocks at intervals along the bank, but this seldom works because
the rock piles just cause much more turbulence in high water, and the river is
likely to eat away the bottom surrounding them, leaving them as isolated rock
mounds in the river. Covering an entire bank with riprap works better, but isn’t
foolproof; there is one spot near my house on the Meramec where the river ended
up eating out the bottom behind the lower end of a rip rap bank, leaving the
line of rock as a submerged ridge in the middle of the river.
Once you succeed in stopping the erosion by hardening the
bank, then you have to plant trees atop the bank to further stabilize the
bottom field, and hope they survive long enough to be of a size to do the job. Often, they do not. It is an expensive, labor intensive, long
term project.
Even if an entire bank isn’t gnawed away, changes in the
flow regime caused by removing trees or just digging around in the bottom field
behind the tree line can cause the river to widen and shallow. One of my favorite places to catch smallmouth
when I was a kid was a quarter mile long pool that was narrow, 4-7 feet deep
throughout its length, flowing between alluvial banks lined with big trees,
with old wood cover along the banks.
With the channel so narrow and confined within high banks, the upstream
portion had enough current and depth to make it a perfect spot for smallmouth,
with big, slick logs in 5-7 feet of water providing the cover for ambush
points.
I fished that pool for 15 years, and it remained completely
unchanged the entire time. And it was
probably just as unchanged for many years before, because my dad told me he
fished it when he was younger and it was always good. Then, the landowner on one side cleared the
trees off his side and began digging and selling topsoil out of the field
behind where the trees had been. It was
not on the outside of a bend, so the current didn’t start cutting his bank
away. Instead, the next couple floods
lowered the bank and allowed water to spread out over the bottomland where his
digging had been. The whole bottom field
was eroded, the bank lowered into a gravel bar, and the upper half of the pool
went from a narrow channel 5-7 feet deep to a wider channel from inches to 3
feet deep…all without the spot receiving more gravel.
So it becomes obvious that humans removing trees is a huge
cause of bank erosion and widening of the channel. But in recent years, we are seeing whole, apparently
healthy, tree-covered banks being torn up and lost to erosion in floods. Having been on these rivers (and being an
observant and curious river rat) for going on 65 years, this is by far the
worst I’ve ever seen the banks on my most familiar rivers. I never used to see whole rows of trees being
ripped out of the banks, nor so many fresh downed trees in the river that long
stretches of good water are unfishable because of all the tangles of trees. What is different now from 40 years ago?
First, we have to discuss the mechanisms of floods and flood
damage a bit more. We all know that
floods cause the damage to banks, eroding them and thus widening the
river. But these rivers have experienced
floods since the last ice age. Floods do
not ordinarily damage healthy banks; if they did, the rivers would have long
ago become as they were back then, wide, braided gravel channels covering the
whole valley bottom. The reason this
hasn’t yet happened is because the power of a flood, as impressive as it is, is
diffuse; spread out over a whole bank from top to bottom. If the bank is healthy, covered in
vegetation, tied together with tangled tree roots, it can withstand the
flood. But give the flood a weak point
to attack, and it will begin to eat away that point. A piece of bare, vertical mud bank, devoid of
roots, is enough if the mud is already wet and loose. Once it begins to gnaw into that weak spot,
it enlarges it, undermines the edges, and rips some of it away.
But Ozark floods drop almost as quickly as they come
up. Ordinarily, the flood might not have
time to excavate much of that bank. The
scar recovers, perhaps gets some vegetation growing on it. It becomes healthy enough to withstand the
next flood. But only as long as that
next flood doesn’t come too quickly. And
only if the wound gets the chance to heal itself.
Which brings us to the two reasons I believe floods in
recent years have done so much damage.
First, I’ve already noted that there have been more big
floods in the last 55 years by far than in the previous 55 years. But there is another component; there are
simply more frequent floods some years, with only short periods of time between
them. The banks never get a chance to
heal before the next flood strikes, nor does the alluvium have time to dry out. One flood starts the damage, the next one
rips out some trees, leaving big scars, and then another one comes along and
rips even more of the bank away.
So it is the frequency of flooding that is different. But there is another difference. I cannot prove this myself, and as far as I
know there have been no scientific studies confirming it. But jet boat wakes are the other wild card in
the mix.
Jet boats first began to show up on the larger Ozark streams
in the 1970s, but did not really become popular until the 1980s. By the late 1980s, some of us old timers (who
weren’t nearly as old back then) began to notice more changes on rivers like
the Current and middle Meramec. The
riffles seemed to be getting wider and shallower, with less well-defined
channels. Places with fast water and alluvial
banks were beginning to see more bank erosion after floods even at that point.
Why? Think about
this. All you have to do is visit the
middle Meramec on a summer weekend to see a likely cause. There will be jet boats going by you, one
after another, a hundred or more wakes pounding the shorelines each day. And these wakes will be causing any shoreline
that is not covered in rocks to spew mud into the river, gradually extending a
mudline until by mid-afternoon the whole river is much murkier than it was that
morning. All that mud is being chewed
off the banks by the incessant wakes. That
is a lot of mud, and it is all coming from that one narrow zone of shoreline,
just a few inches above and below the waterline. It is an indication that the zone along the
waterline is being damaged by the wakes; the mud is not appearing out of thin
air. It is being removed from the banks,
and that is wearing small weak points in those banks. In itself, it is not doing any obvious
damage, but it is giving the floods those small zones to attack that wouldn’t
have been there without the wakes. And
once the weak points are attacked by the flood, the same dominoes fall; the weak
spots expand, a whole section of bank is weakened, and the damage spreads.
And as the banks erode, the river widens. A wider channel flowing the same volume of
water is a shallower channel; no excess gravel necessary.
And there is one more major factor in the pools of the Ozark
streams filling in; the gravel in the channels is moving more than it
should. A natural gravel bar is somewhat
stable; huge floods will move it, but smaller floods may not. Small trees like willows, maples, and
sycamores, water willow weedbeds, and other vegetation grow on gravel bars and
help stabilize them, but even a bare gravel bar is usually somewhat
stable. The gravel is weakly cemented by
silt on the surface in what the experts call “armoring”. Drive a vehicle out onto a gravel bar, and as
long as you go slowly and don’t spin the wheels, the tires will usually stay on
top of the gravel. But if you break
through that surface armor, the gravel beneath is far looser and less stable,
and you immediately sink to the axles.
So anything that disturbs that armoring is going to make the
gravel move more extensively in floods.
A major culprit, especially on smaller streams, is ATV use. It has been
illegal to drive ATVs into, across, and up and down stream beds for a couple
decades now, but the law is widely ignored.
Take a look on Google Earth if you don’t believe this. You’ll see ATV tracks covering gravel bar
after gravel bar on just about any stream with big gravel bars. On some nearly dry tributaries, you’ll see
tracks going for miles down the stream.
Those tracks are an indication the gravel bars and stream bed are not in
any semblance of stability. Floods will
move that gravel. Even a small flood
will pick it up and deposit it at the next spot the current slows—which will
usually be the next deep pool. A huge
flood might pick it up and deposit it all the way out in the bottom fields, but
numerous smaller floods just move it downstream in the channel.
That’s where we are now.
We are seeing the channels filled in with new gravel, ice age gravel
being excavated by bank erosion, “legacy” gravel already in the system moving
downstream and filling in pools, and it’s all exacerbated by bank erosion
widening the channels. The gravel is
mostly natural, but it has become more harmful because of various human
activities.
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