howdy everyone and thank you for continuing on with module 7 in this module we're going to continue looking at rivers by looking at some of the beautiful landforms in which they form and some of the locations of where we can expect flooding to occur so there are two different types of stream channels there are braided stream channels which is shown here on the left um these don't really have a main channel or at least it doesn't have a consistent main channel these usually form in high gradient areas so mountainous terrains and they usually carry large coarse grain sediments um at flood stages but then they drop the that sediment load whenever it's not in a flood stage so we have a lot of deposition of sediment um so this causes the intersecting maze of these smaller channels in these channel bars however we usually are more familiar with the meandering streams especially here in texas where we have a pretty well- definfined main channel these are usually found in low gradient areas so flatter terrain um and the main sediment load that these streams are carrying are tend to be a lot finer grain than our braided streams and we're going to focus most of our time talking about these mand meandering streams because this is what impacts us probably the most but i can't not talk about braided streams because they are so beautiful and they do impact people that live in these high mountainous terrains this is a braided stream in alaska um you can see here this exists in between these two mountain ranges in this valley and this actual valley here is what we call a flood plane so when this reaches flood stage you could have this entire valley underwater which is kind of mindboggling this this is this is what streams and rivers can do in large events now this is a smaller scale one this is still a braided stream in the wasuch mountains of utah now during flood stages it does inundate the land completely underwater but the velocity of the water also increases and it carries these larger gravel pieces but we're not on flood stage so it drops those gravel pieces and that's what forms these gravel bars that you see and here's just another spectacular example of a braided stream in southeast iceland but again we're going to focus most of our time on meandering streams so you can see with a well- definfined main channel and it's literally just moving back and forth across the landscape it's meandering through the landscape here's another meanding stream in cambodia you can see the main channel here you can see some other features that we're going to talk about so right here this is called a cut bank where the water's cutting the bank on the outside bend of the meander but totally different looking on the inside bend this is called a point bar so the velocity of the water actually increases along the cut bank here so it erodess the bank but the velocity slows down on the inside bend so it drops its sediment load over here that's a point bar and you can see that on almost all of the bends in the rivers the point bar is on the inside cut banks on the outside and some other features in the landscape this is an ox bow lake over here is another oxal lake this is because the river used to flow this way and it used to flow this way but of course streams are very dynamic they do not remain their current positions forever they are constantly evolving this one is one in ohio this is the kyhoga river and again you can see those point bars and cut banks very nicely on either meander either bend of the stream now these channels can form these intricately looping curves when the land surface gradient is low so when we have our flat land we can get these really wide swooping meanders and during a flood they over overflow their banks and they can flood the entire valley the entire flood plane but these meanders going back and forth through time sometimes hundreds to thousands of years is what's actually cutting into the landscape and forming the flood plane itself so you can see at the beginning of our river formation over time you can see it's going back and forth creating a much wider valley much wider flood plane until like present day this is how we get those flood planes actually form so during a flood the entire flood plane can be immersed and underwater and that's going to form some natural levies that form some uh ridges parallel to the channel that we'll see here in a bit so here's just the many features of meandering streams again the flood plane is the area around river channel that gets covered by water during a flood so it's usually in between these two bluffs the channel is that trough in which the river normally flows through and then the natural levy is that sediment on either side of the channel that gets accumulated every time the water over tops its banks it's dumping out sediment so over time those banks get steeper and steeper and it's almost like na nature's way of protecting us from a flooding event it'll take a higher water level to over top those banks the next time so here's a picture of a flood plane that you can very clearly see because of the agriculture the fertile soil that exists in the flood plane the flood plane is the valley itself between these two mountain ranges and you can see this green squiggle line this is where the stream actually is where it's lined by those trees but the flood planes that whole area um between the two bluffs here's another flood plane in british columbia you can see the entire valley is the flood plane this entire valley is at risk of flooding events okay so more information on those natural levies um these natural levies are raised areas on either side of the channel that forms when water over tops its banks and they flow they flow out like flat sheets so you can see here this probably shows it very nicely this is during a flood the water's flowing out like flat sheets um and then it builds up those sediments over time so each flood you're building up more and more layers of um sediment so that those banks get higher and higher so the next flood will have to be larger in order to over top its bank so again it's like nature's way of trying to protect us from floods now here's just another diagram we have lots of natural levies in the mississippi river um this is exactly how they form so here's our river channel we have our levies already there during a flood over tops its bank it's going to dump more and more sediment make those banks a little bit steeper now here are those point bars that i was talking about point bars are the inside bend of the meander so they're these sand or um silt sometimes mud deposits on the inside bend of a stream and you can see them very clearly um i my favorite one of my favorite things to do on an airplane is look out the window looking for meandering streams and then you can literally point out the the point bars and they're they exist at seriously every single bend of the river every single bend of the river you'll see this point bar on the inside bend and then we have the cut bank on the outside bend and it all has to do with velocity and how the wa the water flows around those bends the water slows down along the inside bend dropping it sediment it speeds up on the outside bend cutting into the land and then we have deltas which is some of the most beautiful features on earth that we can see from space this is one beautiful example this is when the river reaches a large body of water it reaches the end this is the mouth of its river when that river reaches the end dumps all that water into the main body the velocity slows down a lot so it drops a lot of sediment with it right there that creates these beautiful patterns now deltas consist of the sediment that's been carried all along the the river stream um when that water enters standing water of the larger body of water again it's the velocity it slows down that sediment drops it drops right there now just like our meandering rivers how they're sweeping back and forth the landscape over time and they never remain constant our deltas are also evolving constantly over time now the main channel feeding a delta can actually jump to a new location based on how the river is meandering and flowing through the landscape because it always wants to establish a shorter path to the basin it's just like us it wants us to take the path of least resistant the easiest way to finish an assignment right i know not all of you are like that but our rivers like to take the the fastest um route to the mouth so that's why our meanders are constantly sweeping back and forth and our deltas are constantly moving as well and this is showing the mississippi river which actually has a series of seven deltas that have formed over the past 7,500 years here's where the delta is now we call this the bird's foot delta and one can guess why we call it a bird's foot delta but that wasn't where the delta's been um its whole time so previously the delta was located over here before that it went over here before that it was all the way over here before that it was all the way back over here again before that it was over here before that it was over here so living on a delta is kind of complicated it's a very fragile environment it's constantly changing now our current delta the bird's foot delta is only 500 years old and it's running through new orleans now you can imagine the type of problems it's going to create if this delta decides to move and that's why geotechnical engineers have built structures to prevent this river from moving any further now abandoned delta loes where the delta has switched locations the old location begins to be starved of sediment so the sediment that was dropped previously starts to compact and lose its water and sink into the land because there's nothing else is being fed on top of it and these abandoned delta loes eventually sink low enough to be submerged under the sea and subsidance is a huge problem for cities that are built on deltas like new orleans because sea level is also rising so new orleans is a great example we have it built on a delta and if we're starting to engineer and try to prevent this river from moving and dumping out its sediments it's going to start to sink so city built on deltas is going to sink because it's a lot of loose sediment that's going to compact over time and that's going to be exasperated if the sea level is also rising at the same time your land is sinking so again new orleans is a terrible place when we're talking about flooding because of this process of subsidance being built on a delta plus the sea level rising so if engineers have prevented new orleans from becoming an abandoned lobe then why is it still subsiding well this also has a role to play with our human modification so yes we are preventing it or trying to prevent it from subsiding we're trying to prevent the delta from moving but we've also done a lot of things to the mississippi river there are so many dams upstream the mississippi river which we'll get into dams at the end of this module but dams they hold back water and prevent flooding but they're also holding back sediment so that means those dams are decreasing the amount of sediment that's being deposited at the delta so we're starting to get our delta starting to get starved of sediment even though it's not moving so that's why new orleans is sinking even though we have prevented the delta from moving because we're still preventing sediment from being transported by the river so again that's why earth as a system is so important to understand you can make changes to one part of your system miles away 100 miles away and it's still impacting your system over here so it's important to understand how these systems interact with each other if we want to understand the consequences of certain actions on earth's system