River

• A drainage basin is an area of land drained by a river and its tributaries with a boundary (known as the watershed), which are usually hills and mountains.
• A river is made up of three sections: the upper course, middle course and lower course.
• The upper course of a river starts as many tributaries which are narrow and v-shaped, each tributary is small with a low volume of water, but combined they fill up the river channel further downstream.
• The sides of the river in the upper course are like a valley, with a large gradient either side which causes the water to run into the tributary.
• In the middle course of a river, the tributaries merge together to form a channel which is rounder in shape and deeper than the upper course, the water here has more energy as there’s a higher volume of it, meaning more erosion takes place, widening the channel.
• The area around the river channel in the middle course is flat and low-lying, which is the floodplain if the river needs to flood.
• The lower course of a river is its last stretch before the sea, carrying the largest volume of water in a very wide and very deep channel, there are ridges either side of the river banks called Levees.
• Often, the worst flooding occurs across the floodplains where the land is flattest and so the flood water travels the furthest.
• A subdued flood could take up to a week of consistent rain.
• A flood can be described as either flashcard>
• A flashcard>
• If the river channel isn’t large enough to contain this excess water the river overspills into the surrounding land.
• Human factors that increase the flood risk include urbanisation, the rate and volume of rain, and deforestation.
• In a subdued flood, because the ground is saturated any rain runs straight into the river, causing longer term flooding but not sweeping people or buildings away.
• Flooding occurs when an excess amount of water fills the river in a short period of time.
• Storm hydrographs represent the variation in the river’s discharge within a short period.
• Features of storm hydrographs include peak precipitation, rising limb, peak flow, lag time, and falling limb.
• Physical factors that increase the flood risk include geology, the capacity of the river, and topography.
• The speed the water travels at is dependent on which course of the river it is travelling through.
• Friction occurs between the water and the riverbed which slows the water down, the more contact the water has with the floor, the more friction occurs which means the water travels slower.
• Floodplains are made from silts which make the land fertile, which is why lots of farming takes place in the lower course of the river.
• The neck of the meander will eventually break (normally due to a flood), creating a straight river and a bend where water is slow if not stationary.
• Every time the river floods, sediment is deposited on the top of the banks and so the levees grow in height.
• Levees are higher than the height of the water but aren’t very wide, since it takes thousands of years for the sediment to build up.
• Estuaries are formed at the mouth of the river (where the river joins the sea), where the river water is affected by the tides as well as the flow of water behind it from the river.
• Since the river flow becomes disrupted and slowed greatly by the tides, there is a large amount of deposition, which can create mudflats and saltmarshes that over time can build into permanent habitats on the river/coastline.
• The thalweg is the path of least resistance on a river.
• If the river floods over the levees, the floodwater can reach out far across the flat floodplain.
• The old meander becomes separated from the main river as material gets deposited at the top, creating the separate ox-bow lake.
• Gradually, erosion bends the river so that the meanders travel towards each other.
• Erosion happens when the fastest water hits the sides of the meander, whereas deposition occurs on the inside of the bend, where the water is slowest.
• An ox-bow lake is formed due to a combination of erosion and deposition.
• In the upper course of a river, the channel is shallow and so most of the water passes the riverbed, slowing the rate of flow due to friction.
• Dredging is a cheap hard engineering strategy.
• Even if a river is managed, extreme weather may occur causing the river to flood.
• Channel straightening and flood relief is an expensive and disruptive strategy.
• The river is straightened and relief channels constructed to bypass the meanders in a process known as channel straightening and flood relief.
• Dredging does not alter the look of the river.
• Temporary flood barriers can be put up, which act like temporary banks and increase the river’s capacity.
• Valuable possessions are moved upstairs and locals are told to stay inside and upstairs.