howdy everyone and thank you for continuing on with module 7 in this video we're going to focus more on the river dynamics and how water is being transported so discharge is the volume of water that passes through a given location of a river per unit of time and it is really important because that's what scientists are measuring at these stream gauge stations all across a lot of the major rivers especially here in our country that helps us predict um flood levels and flood watches and warnings for communities that live along these rivers so they're constantly monitoring the discharge again the volume of water passing through a g given point at various locations along a stream to try to predict some flooding events and how do we calculate discharge is discharge is usually represented by capital Q that equals the width of our flow times the depth of our flow times the velocity of our flow so it's the volume of water so um we have our width time depth so the volume and that is through a given point of a river per unit of time that's where we need our velocity at so you can calculate it if you know the width the depth or the cross-sectional area of your stream times that by the velocity of your flow and you might be asked to do this on a quiz or an activity so this is a really good video to watch beforehand so this is showing how many places in our country the USGS is operating and maintaining stream gauge stations and they have over 7,500 stream gauge stations and you can see here they're constantly monitoring that discharge the stream flow and they have they designated different colors depending on what type of level so all of the red we're showing low stream flows this happens during drought events um and then we have black and blues that are above normal um that is during flooding events so you can see here there's a nice correlation with some of our rivers because we have precipitation our water sheds all that water is being dumped into certain rivers causing the levels to rise along the entire river all right I have another thought exercise for you as you get ready for your quiz this is a really good example um this I'm going to ask you maybe to calculate a discharge so this is showing you how to do that remember discharge is the cross-sectional area times by the velocity of a stream so width time depth or just area time velocity and here I have shown a stream in this diagram that's flowing at an average velocity of 6 ft per second the area the cross-sectional area of this dashed line is 800 square ft what does that make the discharge remember discharge cross-sectional area times velocity 800 * 6 you can use a calculator if you need to what does that make it hopefully you got C right 6 * 800 is 4,800 cubic feet per second that's what the discharge at this location on this stream is now during a flood our discharge is going to increase this is why scientists are monitoring the discharge during a flood that discharge increases that causes the flow to exceed its stream channel and then we begin having moving water and debris scouring the flood plane now the velocity of the flowing water is going to get slower as you move away from that main channel and as the velocity slows remember that's going to drop our sediment so sediment's going to get dropped when our when our water slows down and that forms natural levies which we'll get into in our next video we'll see some pictures of that but that's just where we have sediment being built up along a channel now notice here that there is a lag between peak rainfall and peak stream flow which is our discharge so here in blue we have precipitation you can see here at 6:00 was the peak of our rainfall event however if you look at the stream flow we don't see a peak in stream flow until almost 10:00 so there's almost a 4hour lag time between peak rainfall and peak stream flow and that's because it takes time for that water to run off the surface make its way to its watershed and those rivers and streams where we get our highest flow point right so that makes sense and lag time is actually good because we're measuring the discharge of our streams were able to send out flood watches and warnings in advance and evacuate people we had 4 hours to evacuate people in this case however when there isn't a lack time that becomes deadlier that's what we call flash flood that we'll get into later on in this module