 The Reynolds number describes the laminar versus turbulent properties of the flow. So the amount of turbulence depends on the flow speed of the fluid, the density of the fluid, and the viscosity of the fluid, as well as the geometry of the flow itself. So we have the Reynolds number, and I'm going to use a version of it that is for an open channel flow with a particular depth. There's a variety of different versions of it. But the Reynolds number is usually RE, and it depends, as I said, on the flow speed. So U equals the flow speed times L, which is the depth of the flow, times the density of the fluid, and all of that is divided by the viscosity of the fluid. So viscosity is a measure of how much friction is within the fluid, like the molecules of the fluid rubbing past each other. So a good example of this is the viscosity of air is very low. When we move through air, we can do it without very much effort. In contrast, water has a higher viscosity, and walking in water is a lot more work because the water molecules have those hydrogen bonds that connect them, and to move through water you have to pull apart those hydrogen bonds, and that increases the viscosity. Ice has an even higher viscosity, and it's high enough that we can walk on it and not through it, and that's because in ice the water molecules are in crystal forms with much stronger bonds. Ice can still flow, but it's at a speed that's very long on our observational time scale. When you get to rocks, rocks can also flow. The ones in the interior of the earth that are at a high enough temperature and enough pressure, those flow on an even higher, even longer time scale, and they have a higher viscosity than ice. So if we can take these Reynolds numbers, this Reynolds number and we can measure it for a given flow, and then we can also interpret that in terms of whether a flow is laminar, transitional, or turbulent. So the boundaries that we're going to use are for an open channel flow. So if the Reynolds number is less than 500, we say it's laminar. And as you know from the last video, that means that the particles are mostly moving parallel to each other. A fully turbulent flow is one where the Reynolds number is greater than 2000. And the particles move in all sorts of directions. And then of course in between we have our transitional flows and not surprisingly transitional flows are ones when the Reynolds number is between 500 and 2000. So this Reynolds number gives us a way to quantify how much turbulence we have and that amount of turbulence affects the transport of sediment. So thanks for watching.