River Processes PDF

RIVER PROCESSES  Drainage Basin ► Area of land drained by a main river and its tributaries ► A river starts in a high-lying area (SOURCE) and flows downslope (COURSE) until it eventually reaches the sea or a lake (ESTUARY / MOUTH) Features ► Source – where a ri...

RIVER PROCESSES  Drainage Basin ► Area of land drained by a main river and its tributaries ► A river starts in a high-lying area (SOURCE) and flows downslope (COURSE) until it eventually reaches the sea or a lake (ESTUARY / MOUTH) Features ► Source – where a river starts ► Tributary - a small river or stream that flows into a larger river ► Confluence - where two rivers of the same size meet ► Estuary - where a river flows into a lake or the sea ► Watershed - area of high land that separates drainage basins  The Hydrological Cycle ► Sun heats earth’s surface and evaporates water from oceans, rivers, lakes etc. ► Warm, moist air is lighter so it rises ► It cools and condensation occurs, forming clouds, which leads to precipitation e.g. rain, snow ► Some water lands on vegetation (interception) ► Some evaporates from leaves or is absorbed by plant roots and is turned into vapour through transpiration ► Prevents some water flowing into river ► Some water flows across the ground as surface run-off. It can’t sink into the ground because it is saturated (full of water) or the rock is impermeable ► Moves quickly to river ► Some water infiltrates (sinks) into the soil and moves slowly to the river as through flow ► Some percolates deep into the ground and moves through the rock to the river as groundwater flow The Hydrological Cycle How water reaches a river Characteristics of Rivers ► Channel - path of river as it flows ► Bed – bottom of river channel ► Banks – sides of river channel ► Depth – distance from water surface to bed ► Width - distance between the banks ► Wetted perimeter - length of bed and banks in contact with water in channel ► Velocity – how fast water is moving. Measured in metres per second ► Discharge - volume of water at a point of the river. Measured in cubic metres per second ► Load - material transported by a river ► Rivers get wider and deeper further away from the source. Velocity, discharge and load also increases.  Types of Flow ► TURBULENT flow - horizontal and vertical circles. Most common. ► LAMINAR flow - horizontal movement, travels over sediment without disturbing it. HELICOIDAL flow - corkscrew or spiral movement  THE WORK OF A RIVER A river uses its energy to do certain things. ► Erosion – wears away bed and banks, and breaks up rocks it carries ► Transportation - picks up and carries material as it flows ► Deposition - when it loses energy, it drops some material ► The greater the discharge, velocity and gradient, the more energy a river has ► And the more it can transport and erode EROSION ► Four processes : Abrasion / Corrasion Attrition Hydraulic Action Corrosion Attrition ► As material on bed is moved downstream, rocks COLLIDE with one another ► Rocks break into smaller pieces ► Become smoother and rounder Abrasion/Corrasion ► Rocks RUB along the bed and banks, wearing them away like sandpaper ► Main way river erodes vertically (deeper) and laterally (wider) ► Most during flood Hydraulic Action ► FORCE of the water pushes water into cracks in the banks ► Air in cracks is compressed and then released, breaking the rock apart Corrosion ► Weaker, softer rocks (limestone and chalk) DISSOLVE when they react with acids in water Types of Erosion Direction of erosion Vertical erosion ► down cutting, valley deepens Lateral erosion ► horizontal undercutting, valley widens Headward erosion ► river extends upstream at source (gets longer) TRANSPORTATION ► Greater velocity = more material (amount and size) can be carried ► 4 processes ► TRACTION - large, heavy rocks are rolled along bed. Common near source - large material carried. ► SALTATION - smaller rocks are bounced along bed. Common near source. ► SUSPENSION - lighter sediment is suspended (carried) within water. Common near mouth. ► SOLUTION - transport of dissolved chemicals. Where soluble rocks are. Methods of Transportation DEPOSITION ► When discharge and velocity drops, a river doesn’t have enough energy to carry its load ► Heaviest material deposited first ► Mostly in middle and lower course ► Most at mouth The Zambezi River The Source Kalene Hill, Zambia Upper Course Middle Course Meanders and Oxbow Lakes Lower Course  RIVER FLOW TRACTION SALTATION SUSPENSION SOLUTION Stones roll Sand-sized Fine particles Some minerals along bed particles carried within dissolve in water (needs most bounce along water flow (needs least energy) bed energy)