So I'm fascinated by the processes by which a river actually works. I've been studying it a lot. Unfortunately the vast majority of things written on streamflow are written so no one (including the authors I suspect) can actually understand just what the h%$# is being said. Take for instance this sentence from a study I just read today: "Incipient accumulations of coarse particles may be perpetuated by altering the flow conditions which influence bed load transport". Or even better this one from another study: "It is further suggested that spatial differences in the near-bed turbulence field arising from incipient riffle—pool topography themselves create differences in surface sediment entrainment which enhance and maintain the sequence in a form process feedback mechanism".
Really? Really?
So anyways, here's the best I can do to explain the amazing processes at work in the river with a minimum of b%$##^%t. Rivers are basically the result of two old time everyday sayings. The first is the Butterfly Effect: This effect grants the power to cause a hurricane in China to a butterfly flapping its wings in New Mexico. The second is the ever popular: Sh%t rolls downhill. Let me explain...
We have an imaginary straight new river. Imaginary because well, the place is billions of years old and there are no new or straight rivers in existence. But imagining one will help you understand how it all works. Our imaginary river is flowing straight. The fastest water is right in the middle just under the surface because the bottom and sides of the river drag a bit on the earth. Well then some prehistoric muskrat builds a den in the bank that collapses, or a tree falls in the river, or a caveman digs in the bank to find flint, or well, you get the picture, something happens to mess with those perfectly straight sides. Now our flow isn't even down the stream, a bit is diverted more towards one side and less towards the other. This digs out the bank a bit on one side changing the flow even more downstream of that which changes the flow downstream even more which changes the flow downstream of that even more, which...well you get the picture. Little things add up to have big consequences, each downstream of the next.
Which is how we eventually end up with the amazingly complicated but somewhat predictable set of rules that govern how our stream flows work.
You see water is flowing down a straight section of river evenly in what we call laminar flow. Slow water dragging evenly along the bottom with faster layers above. Then this is diverted and it flows across the river and slams into the far bank. This even laminar flow hits the bank and curves under itself and then spirals downstream. Always bending under itself and spiraling down the river in what it called hellicoidal flow. It might help if you imagine a huge slinky lying in the outside curve of a bend in the river. If you could see the flow that is kinda what it would look like. And since our spiraling current is is going down the bank and across the river bottom back towards the inside of the bend with each spiral it digs out the bank even more and sweeps sediment back across the river towards the inside of the bend. This builds up a rock and sand bar on the inside of the bend called a point bar.
So our bend digs out even more as the river has to rush around this bar and the river over time just curves even more. The water then comes whooshing around the bend and runs diagonally across the river and downstream to crash into the bank down there and create the next bend and point bar on the opposite bank. So you have a series of deep bend pools and point bars alternating sides down the river. And our spiraling current straightens out between each to only hit the bank and spiral underneath itself again, spiraling clockwise on one side of the river and counter clockwise on the other. In most really good smallmouth streams the distance between these bends is roughly five to seven times the width of the stream. The channel the main flow of the river follows down the river is called the thalweg and also runs diagonally across the river from one bend across the river to the next.
So our spiraling current as it goes around the bend mostly carries smaller stuff back across the stream to build up the point bar. The big stuff dug out is just too heavy to be swept back across the bottom. Instead in times of high water or flood the water raging around the bend picks this bigger material up and carries it downstream. It then drops it roughly halfway to the next bend. As this material is dropped over time it creates a riffle. A riffle is by definition a shallow extending across a stream and most are built up this way.
Now the flooding river carries bigger material out of the pool to form the riffle which shallows the river speeding up the current which in turn sweeps away finer material that was deposited along with the larger stuff. Which brings us to a very important concept to understand as fishermen. Which is that the composition of the bottom is not even along a stream but instead has the biggest material on average at the shallowest part of the riffle tapering down in each direction to ever smaller and finer material the further we get from the riffle and the closer we get to the bend pool.
For you see our smallmouth streams have a very very complicated food chain which is many more times more complicated than that of a trout stream. Take the river I know the best, the Little Miami River(LMR). The LMR has about a dozen shiner species, a dozen darter species and all kinds of chubs, regular minnows, madtoms, and well all together just a whole heck of a lot of little fish. Why? Because over time all those little fish have adapted to use the river in different ways. Some like darters live in the cracks between the bigger rocks of the riffle, some just below a riffle, some in the pools and some in quiet backwaters and everywhere in between. The same with the rivers other life, with things like crayfish and net spinning caddisflies and hellgrammites in the rocks and others like mayflies burrowing in the soft bottom of the pool and on and on. Every different type of water in our streams has something best adapted to use it. And that something is usually bass food or at least feeds something that is bass food.
Now in a perfect (and possibly boring) world our stream would just alternate one bend after another in symmetry but thankfully our world is anything but boring and symmetrical. Instead we have things like feeder creeks, bridge abutments, different sized rocks, rocks of different hardness which create ledges, an endless list really of things that upset the pattern and change the flow going downstream a bit which in turn changes the flow downstream of that which...well we have been over all that before.
But knowing the basic blueprint the river is following lets you be on the right side of the river to fish the bends easily and lets you know where the next riffle is going to be and that you then need to cross here to set yourself up for the the next bend there, then you...Well, again it all rolls downhill.
Anyways I've rattled on enough for one night, I'll do a part 2 and possibly a part 3 some other night. There are things like lateral sort, boundary layers, current breaks and eddies which mean the world to a fisherman that we haven't touched yet. Hopefully my bad drawings help and not complicate things needlessly
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