 When sediment starts to move, it interacts with the flow very nicely. So for example, we talked about how when we have a grain on a bed that gets lifted into the flow with the Bernoulli effect, the flow pushes it downstream. And so the part of the momentum of the flow goes into the momentum of the grain. And then if it's large enough, it comes back down and sits on the bed again. So this is for an individual grain. And one of the things that happens is there are irregularities on the bottom of the bed. So that's intentionally an irregular line. And let's say that we have a grain sitting right here. It takes a little bit extra to lift it up over this because this high, because the flow is deflected a little bit over the high, but maybe a little bit of flow from this other side can come in and start to move it. The geometry of the flow can change with the geometry of the surface at the bottom. The other thing is we typically have a whole bunch of grains sitting on the bottom as geologists were interested in natural flows rather than things on, say, clean cement surfaces most of the time. So these grains also interfere with each other. So what often happens, I'm just going to draw the grains as dots now. So we have grains saltating and they saltate different distances. What often happens is if you can end up with a little bit of a pile of grains in one place here. And when you end up with a pile of grains, it starts to deflect the flow. So if we have our flow coming in like this, this pile of the grains deflects it upward, and then the flow has some momentum and it often shoots downstream. As it gets closer to the bed again, maybe it sort of digs up into the grains right here. So as you can imagine, that when you have this topography, it influences how the grains move because it's changing the flow characteristics here. So if we draw, this is our pile of grains here. Let's stop it a little sooner than this. All right, so this, I just sort of magnified this here. What happens is our flow is coming along and in this zone right here, our boundary layer is thin. And when the boundary layer is thin, it's easier for the grain is to get picked up. All right, so the Bernoulli effect is more effective when you have greater shear at the boundary layer, which is what happens when it's thin. When the flow reaches the top of the high, it has momentum going downstream like this. And over here, your boundary layer is thick. And when the boundary layer is thick enough, you get a lot less sediment transport. So in this zone here, you get erosion of grains. The effective flow speed is higher. And in this zone here, you tend to have little transport of the grains because that boundary layer is thicker. So if we have a grain here that gets picked up into the flow and it's saltating and it happens to land in the zone with a thick boundary layer, it tends to stay there. So what this is doing is it's connecting the characteristics of the boundary layer to when and how sediment gets eroded or transported along a bed. So thanks for watching.