Theme 1A

• The drainage basin is the area within the boundaries of the watershed drained by a river and its tributaries.
• The drainage basin is in a constant state of change. A systems approach, where there are inputs, transfers, stores and outputs.
• Some river stores include: surface storage, vegetation storage, soil moisture storage, groundwater storage and channel store.
• Some river transfers include: surface run-off, stem flow, infiltration, percolation, through-flow, groundwater flow, transpiration, evaporation and channel flow.
• In a drainage basin, precipitation is the primary input.
• The water outputs the system into the ocean, sea or lake and water is also lost through evapotranspiration.
• The variations in climate, rock type, occurrence of storm events, altitude, relief, soil depth and type, vegetation coverage and human activity all affect the system.
• Annual hydrographs highlight variations in a rivers discharge over the course of a typical year. They are based on typical flow patterns and variations will be determined by the precipitation levels and the temperature.
• There are 3 river processes: erosion, transportation and deposition.
• Erosion includes abrasion, hydraulic action, attrition and corrosion.
• Vertical erosion is the wearing away of the river bed and cutting downwards to deepen a river.
• Lateral erosion is the wearing away of the banks of the river to widen the river channel.
• Transportation methods include solution, suspension and bedload.
• Solution refers to the process of erosion whereby rock is dissolved by river water.
• Suspension is the fine material carried by a river current that is held up or suspended mid-channel.
• Saltation is the skipping or bouncing of small sediment as it travels along the river bed.
• Traction is the rolling of larger boulders and rocks as they move downstream.
• The Hjulstrom curve shows the relationship between the size of the material and the velocity level at which erosion will take place.
• On a Hjulstrom curve, velocity is on the y-axis and sediment size is on the x-axis.
• On a Hjulstrom curve, there are two main lines. the critical erosion velocity curve and the critical settling velocity curve.
• Deposition occurs when there is a loss of energy. This happens when a river slows down, like at a bend in a channel, flooding, gradient and obstacles.
• As a river slows down, it has a lessened carrying capacity and begins to deposit sediment in a graded fashion, starting with the heaviest first as it requires the most energy to erode and transport.
• Levees are ridges of deposited material that are found on the rivers banks in a floodplain. The river floods and rapidly loses energy due to the large surface area to volume ratio and deposits its load in this graded fashion.
• Waterfalls are formed when a layer of hard rock sits on top of the softer rock. Hydraulic action erodes the softer rock and creates a hard rock overhang. This collapses and the rocks create a plunge pool. Over time, this cycle will repeat constantly and the river will retreat upstream and leaves a narrow, steep sided gorge.
• A meander is a feature in the lower course of a river and often in the flood plain. They start with the formation of pools and riffles. The path of fastest flowing water, the Thalweg, swings from side to side. This affects the rivers depth and friction, which result in erosion. Over time, the bend becomes more sinuous and on the inner bank, a river cliff is formed, and on the outer bank, a slip-off slope.
• A pool is a section of deep water with smaller bedload materials, like pebbles. Pools are found where a meander will develop, they dictate where the bend will occur.
• Riffles are sections of shallow water and much larger bedload material.
• An oxbow lake is formed as the result of a meander becoming more and more sinuous until the water can take a shortcut through the neck of the loop. They are created by a repeated process of erosion and deposition.
• Levees and flood plains are often the focus of river management strategies. Levees are often created artificially and maintained as part of downstream river management strategies.
• Deltas are features of deposition at the mouth of a river at a sea or lake. They form due to a sudden decrease in velocity of the river and so a dramatic decrease in the river's carrying capacity. The load it is carrying is deposited in a graded fashion on the river and sea bed. Flocculation also helps with delta development. There are two types of deltas: arcuate and bird's foot deltas.