RIVERS

RIVERS

1. How rainwater reaches the river

   • 1.Some rainwater just runs along the ground. This is called surface runoff.
     • 2.The rest soaks into the ground. This is called infiltration.
       • 3. As it soaks through the soil, some runs down the slope to the river. This is called throughflow.
         • 4. The rest soaks right down, and fills up the pores and cracks in the rock. Now this is called groundwater. (This rock is impermeable. Water cannot pass through it, so no groundwater here. The top of the groundwater is called water table.)
           • 5. Groundwater is always on the move. It flows along slowly.
             • 6. Together, the surface runoff, throughflow and groundwater feed the river.

2. River's Journey

   • The source is where the river starts. It could be a spring, a lake, a melting glacier, or a hollow where a lot of rain collects.
     • A river runs in a valley: an area with higher land on each side.
       • Smaller rivers join the main one. They are tributaries.
         • The point where two rivers join is called a confluence.
           • The flat land beside the river, which may flood when the river overflows is called its floodplain.
             • Rain falling in the area inside the watershed feeds the river. This area is called the river basin/drainage basin.
               • The watershed is the dividing line between one river basin/drainage basin and the next. It’s usually a ridge of higher land.
                 • The mouth of the river is where it flows into a lake or the sea. A wide river mouth into the sea is called estuary. Here, fresh river water mixes with the salty sea water. (The river gets wider from source to mouth, and carries more water.)

3. River's Long Profile:

   • A river flows downhill from source to mouth.
     • Long profile- The side view of a river from source to mouth, showing how the slope changes.
       • You can divide the river into three parts:
         • The upper course, where the slope is the steepest
         • The middle course, where it is less steep
         • The lower course, where it flattens out
         • This river finally reaches sea level.
   • Erosion is the main process in the upper course. Both erosion and deposition occur in the middle course. Deposition wins out in the lower course, as the river loses energy. Eroded material is transported downriver.

4. River's Channel

   • A river carves out a channel for itself.
   • There is a river bank on each side of the river channel.
   • The bottom of the channel is called the river bed.

5. Erosion

   • Erosion means wearing away. The river erodes the land it flows over. How it happens:
     • The water dissolves soluble minerals from the bed and banks. This helps to break them up. This is called solution.
     • Rocks and stones and sand in the water act like sandpaper. They scrape the river bed and banks and wear them away. This is called abrasion.
     • In a fast-flowing river, the water is forced into cracks in the bank. Over time this breaks the bank up. It is called hydraulic action.
     • The rocks and stones wear each other away too. The bang together and knock bits off each other. This is called attrition.

6. Transport

   • Next, the river carries away the eroded material. This process is called transport. The material is called the river’s load. This is how it's moved:
     • Dissolved material is carried along as a solution. You cannot see it. Small light particles of rock and soil are carried along as a suspension. They make the water look cloudy or muddy.
     • The heavier material is carried along the bottom. It is called bedload. The larger stones and rocks roll along. This is called traction. Sands and smaller stones bounce along. This is called saltation.

7. Deposition

   • Then, as the river reaches flatter land, it loses energy. As it loses energy, it drops or deposits its load. The deposited material is called sediment.
     • The biggest, heaviest, stones and pebbles are deposited first. Then smaller ones. And last the smallest particles.
       • But dissolved material stays in the water. It will end up in the lake or sea.

8. The river’s valley, in cross-section

   • In the upper course, the river erodes downwards. The result is a V-shaped valley.
   • The banks then get eroded, so the channel gets wider. A floodplain forms, where the river floods time after time.
   • In the lower course, the channel is wider still, and a little deeper. It holds a lot more water now. The floodplain has gotten wider too.

9. Landforms

   • V-SHAPED VALLEY-
     • In its upper-course, a river erodes sharply downwards, cutting like a knife. This gives a steep valley. Tain washes soil and stones from the side. So over time, the valley becomes V-shaped.
   • A WATERFALL-
     • A waterfall is where water tumbles over a ledge of a hard rock.
     • The waterfall form because there is softer rock below the hard rock, and it is easier to erode.
     • FROM WATERFALL TO A GORGE-
       • The hard rock erodes very slowly. The soft rock below it erodes much faster.
         • Erosion of the soft rock leaves a ledge of hard rock and a hollow called a plunge pool.
           • In time, the ledge falls into the plunge poll. The debris from it helps to speed up erosion.
             • Steps 1-3 are repeated. The waterfall gradually retreats upstream, carving out a gorge.
     • A MEANDER-
       • Water flows faster on the outer curve of the bend, and slower on the inner curve.
         • So the outer bank gets eroded, but material is deposited at the inner bank.
           • As the outer bank gets worn away, and the inner one grows a meander forms.
             • Over time, as the process continues, the meander gows more “loopy”.
       • AN OXBOW LAKE-
         • As erosion continues, the neck of the meander is worn away. It gets narrower and narrower.
           • Then, during flooding, the river takes a shortcut-straight across the neck.
             • Soon the loop of the meander gets sealed off. It turns into an oxbow lake.
               • In time the lake will get covered with weeds, and fill with soil, and disappear.
   • Delta
     • flat, low lying deposit of sediment that may be found at a river’s mouth
       • carry a large volume of sediment.
         • enter a still body of water which causes velocity to fall.

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