Transcript of Fluvial Processes - River Characteristics

our next thing to address um in our lab is going to be the different characteristics of a river and a river Channel um there's many different stages of a river uh as you know water kind of takes its easiest path uh and it's going to go um you know wherever it's it's lowest right water wants to find the lowest spot if you think about if your if your basement's ever filled up with water it's uh always going to pull in lowest spots first um and that's true for River that is Flowing down from a high spot to a low spot as well it's going to follow the valley um so water is going to flow through land where there's already a valley that's cut out it's taking its easiest path um and it's going to uh kind of be fast moving um so fast moving rivers are are newer Rivers um and it's going to follow a relatively straight path as a river gets older and older and older uh we go from just this v-shaped Valley here um to a wider Valley and the river is going to go ahead and start cutting into the land more and more and it's going to be cutting down into the land more and more um and it's working down towards the the water table um and as it's cutting into the valley and cutting down into the land it's going to begin to what we call Meander which means it's going to work back and forth and back and forth and back and forth you're also going to see that as it meanders no longer is it just eroding all the time uh it is eroding and it's depositing as well um in a meander it is going to erode on the outside of that bank um where the water is Flowing the fastest on the inside of it right here um we are going to see some some deposition um so that means that it's going to deposit whatever particles um sand gravel silk clay um that it might be carrying so we have erosion and deposition happening in a relatively Old River and uh the the flood plane is going to be wind out by quite a bit in a very old River we're going to see some much larger meanders we're going to see a l much larger um uh flood plane we're going to see some well-developed Marsh areas as well and we're going to see that that River has really uh really slowed down um it's got a nice um River bed uh it's it's got a nice Sandy and silty River bed versus a a rockier river bed um and that flood plane is going to be again very very welldeveloped so again kind of looking at uh just the general flow and plan of a river um when there's a a gentle slope uh we're going to see that the river is going to move relatively slowly um as those slopes get sleeper steeper the river is going to move faster um it's going to be able to then cut into um the ground that it or Surface that it is flowing over um a lot easier uh we might begin to see these these meanders uh in these meanders we're going to see um that there's going to be that deposition site and also that cut bank site um where erosion is happening so water is moving much faster on the outside um than it is on the inside we're going to see erosion happening out here and deposition happening inside of here and then um you might be able to look at the cross-section of a river as well and here's some um things that we might see a curved channel is going to look like this um you can imagine that this is where the erosion is happening this is where land is getting cut into and then this is where deposition is happening this is where sediment is being laid in a straight Channel um it looks pretty much um uniform all the way across uh there's no one place where it's faster flow and slower flow uh there is no sight of deposition um and there's no sight of of erosion um it's pretty much uniform all the way across and a braided Channel um typically slower moving channels uh where the amount of water that flows through them changes annually or seasonally um and they're going to be relatively shallow but again pretty much uniform all the way across uh again this is just going to show that um during High flows that's when a river is going to be most effective at eroding uh that's because there's more water there to carry more sediment um water is moving faster which means that it has more power to erode um and that means it's going to make that deeper Cut Bank so over time if this is what the riverbed would have looked like if there was a higher flow after it scours it out it picks up all that sediment um it would bring it here and erode it down to a a deeper level and once it um began to slow down again we would see more and more deposition we would see um more sediment build up on that inside deposition bank so so again um we see a an older River we've got a relatively wide flood plane um it's uh pretty welld developed and cut down into its um riverbed and it's got some relatively large um U meanders um and then here is what we've we've got our deposition bank or what we call a point bar Point bars are deposited on the inside Banks and bends along a travel Channel due to lower velocities and increased flow friction so um water is moving slower at this point in a meander than it is during the straight points or out here in the outside of that Meander so we're going to see some nice kind of sandy beaches here um and some nice deposition of sediments so you could imagine that um there's going to be different amounts of sediment uh deposited in um Rivers based on how much it meanders for a river that doesn't have very large meanders this might be a newer River we are going to see um smaller deposition Banks and smaller cut Banks um but on the other side when these meanders are much larger uh we're going to see larger deposition Banks and also larger um cut Banks here braided channels we kind of talked about um braided channels are beautiful um we see them in places where there is a large fluctuation in seasonal or annual flow typically we see this um in the drier parts of the country um or we might see it in um places like the Arctic region so this is very common in let's say Alaska um and it's because of the amount of snow melt and then um during the Spring thaw and then it dries up uh we could also see this in a desert and then um again like we were talking about there's a certain capacity that Rivers have to carry sediment um and that that capacity to carry sediment is going to um be based on how fast that river is moving um so the things that we would talk about is how much can be suspended how much is the river itself actually carrying and then how much is um actually moving across the bed of the river so there's a couple different different types of um of particle movement there's a suspended load stuff that's actually suspended or floating um in in the water itself and then there's the bed load the stuff that's either rolling across the bottom um or just kind of Shifting along its way uh if you went into a stream and were to pick out some rocks you would find that most of them are really well eroded um they have nice round surfaces uh and it's because of this movement of particles through the water um so you can have a suspended load or a bed load we could also have saltation um which is the bouncing of particles um and they're kind of getting picked up and rolled around if you can imagine imagine that but all of these things the the river's ability to do this is based on its flow and flow rate so one of the things you're going to be looking for in a in your lab is uh to determine how different flow rates affect different amounts of sediment that can be carried and the size of sediment that might be carried again these are just some different diagrams showing you um how effective a river may be at transferring that that sediment the terms competence and capacity are used to describe the efficiency at which sediment can be um can be transported so the capacity is how much could it actually carry um and competence um is really how much is it carrying uh all of this really just comes back to talking about how do um human interactions affect uh natural things and um how do rivers work in nature when humans don't try to control them one of those things might be talking about flooding and the formation of of New River beds like you see here