 Braided rivers form when there's a lot of bed load sediment being transported. We're going to talk about this example from these Braided rivers are rivers that have a lot of channels that are separated by bars of coarser sediment. So the image on the right is an expanded view of the channel on the left and There are numerous channels they split and merge in different places in a braided pattern So one of the characteristics of braided rivers are multiple channels but those channels are actually confined into within a larger zone. So this happens to be a Fluvial plane in New Zealand where there's a lot of farmland and the boundaries of the farmland represent the boundaries of the braided river in here. So if we look at a cross section across this river we have the farmland and then this point right here is usually a topographic low and then you have a series of bars and channels various sizes and then you have the the farmland again on the other side and so the channel width here there's there there's sort of two scales one is the whole breadth of the river and Then there are sub channels within that within that larger zone of channels and so you might have for example the river filled up to a level something like this where these are individual channels at the lower flow So the one of the interesting things about braided rivers is the importance of floods in their formation and so a lot of times when braided rivers flood the the water will fill up this whole broad width So this might be the the flood Elevation and when the water is high That's when most of the sediment in the bars and river channels get deposited The sediment that's sitting on top of the bars Here isn't being transported now because there's no flow over it But that sediment is regularly transported at the high flow during the flood The like for example the spring melt floods Sometimes the water will go all the way up flows that go out Into the the overbank deposit area Into the farmland are much rare and we can look at what happens with an individual bar as well. So we have a Bar or something like this With water flowing on either side of it. So the water is coming down here and it divides on either side of the bar and Then let's say that the two channels just rejoin Down here The flow is coming right up at the tops of the bar here. And so you tend to get erosion in this area and Then you get end up with deposition where the flow is Slower in general, which is down at this downstream end of the bar So in the zone down here You tend to get deposition So the bars can also Move sideways. So for example, if you look up here the This channel is flowing and it's turning around and The outside of that channel the flow is going into the bank And that is a place where you typically have erosion. So in general the bars move downstream as well as sideways and That's sort of during what I'll call Normal flow. So the transport of sediment in the channels With the flowing water vary depending on the Reynolds number Which includes the flow speed the flow depth and the density of water and the viscosity of water The flow depth and flow speed change dramatically depending on where you are within the channel a little channel and A little bit of a bar and then a much bigger channel and then it then it comes up And so the water will can be is flowing in both of those Yeah but the flow depth is different in the two channels and it is Different even within a channel and so the Reynolds number here is the highest in terms of the flow depth and The middle of the flow also tends to flow more quickly In part because there's less friction with the bed in this zone here So if we plotted the flow speed Versus distance the flow speed would be Fastest in the middle and it slows down Towards the edges of the channel So we have the highest Reynolds number in the middle of the flow and the highest flow speed What that means is that the grain size of the sediment varies within the channel that all grain sizes can be transported in the middle of the flow those that are Large enough for that flow speed But when you get to the sides the largest grains don't get transported there and the smaller grains can get deposited So basically down in the bottom of the channel We have our largest grains deposited and On the edges of the channel we have our smaller grains deposited and Similarly if you look at this channel the whole thing is smaller There's a lot more friction and so the average flow speed in this channel is a lot lower and So this this channel will have again a smaller grain size relative to The the large channel so some of the characteristics that we have for braided rivers as we have our Multiple channels within a larger channel And then the overbank areas outside of that large channel We have the bars that migrate downstream during normal flow We have a lot of variability in the grain size that depends on the flow speed within an individual channel, but also depending on the size of the channel and then we have the aspect that floods and The flows that fill up the larger channel Are the ones that really shape the geometry of the river Braided rivers form when there's a very high proportion of bed load sediment to create these bars and At the same time the soil has to be high enough that the river can actually have The transport capacity to move some of that sediment In the normal channels as well as in the floods. So braided rivers have a lot of variability Because the the flow speed is highly variable the grain size is highly highly variable And there's enough energy from the flow down the slope to Maintain the river flow. Thanks for watching