RIVERS

FORM 5

GEOGRAPHY CLASS

2011

OBJECTIVES FOR THIS CLASS

• THE WATER CYCLE - HOW WATER FLOWS ON REACHING THE SURFACE

• PATHWAYS WATER TAKES ON REACHING THE SURFACE

• THE DRAINAGE BASIN AND THE RIVER’S DRAINAGE PATTERNS

• RIVER PROCESSES: EROSION, TRANSPORTATION AND DEPOSITION

• STAGES OF THE RIVER AND THE LANDFORMS AT EACH STAGE

WHAT IS A RIVER?

• A LARGE NATURAL STREAM OF WATER FLOWING IN A CHANNEL TO THE SEA, A LAKE OR ANOTHER STREAM.

SOURCES OF A RIVER?

• AN UPLAND LAKE

• A MELTING GLACIER

• A SPRING IN AN UPLAND AREA WHERE THE SOIL IS SO SATURATED THAT SURFACE FLOW BEGINS

• A SPRING AT THE BOUNDARY BETWEEN PERMEABLE AND IMPERMEABLE ROCKS

THE WATER CYCLE

• IS ALSO KNOWN AS THE HYDROLOGICAL CYCLE.

• IT DESCRIBES THE CONTINUOUS MOVEMENT OF WATER ON, ABOVE AND BELOW THE EARTH’S SURFACE.

PROCESSES OF THE WATER CYCLE

• EVAPORATION – THE PROCESS BY WHICH WATER IS CHANGED INTO WATER VAPOUR (A GAS)

• TRANSPIRATION – THE EVAPORATION OF WATER INTO THE ATMOSPHERE FROM THE LEAVES AND STEMS OF PLANTS

• CONDENSATION – THE PROCESS BY WHICH WATER VAPOUR TURNS INTO WATER DROPLETS. (THIS HAPPENS WHEN WATER VAPOUR IS COOLED) AND CLOUDS ARE FORMED.

• PRECIPITATION – THIS OCCURS WHEN WATER IN ANY FORM FALLS FROM THE ATMOSPHERE TO THE SURFACE. (RAIN, HAIL, SLEET, SNOW).

PATHWAYS WATER TAKES ON REACHING THE SURFACE

• WHEN PRECIPITATION REACHES THE SURFACE IT CAN FOLLOW A NUMBER OF PATHWAYS…

• THEY ARE AS FOLLOWS …

• AN SMALL AMOUNT OF WATER FALLS INTO RIVERS AS DIRECT CHANNEL PRECIPITATION. THE REST FALLS ONTO VEGETATION OR THE GROUND. IF HEAVY RAIN HAS FALLEN PREVIOUSLY THE SOIL IS FULL OF WATER AND IS SAID TO BE SATURATED.

• BECAUSE THE SOIL IS UNABLE TO TAKE ANYMORE WATER, THE RAIN FLOWS ON THE SURFACE AND THIS IS CALLED SURFACE RUNOFF OR OVERLAND FLOW. IF THE SOIL IS NOT SATURATED, RAINWATER WILL SOAK INTO IT, AND THIS IS CALLED GROUNDWATER FLOW.

OTHER KEY DEFINITIONS…

• WATER FLOWING THROUGH THE SOIL IS CALLED THROUGHFLOW. INFILTRATION IS THE PASSAGE OF WATER INTO THE SOIL. PERCOLATION IS THE DOWNWARD MOVEMENT OF WATER WITHIN A SOIL OR ROCK. WATER CAN ALSO BE INTERCEPTED (INTERCEPTION) BY THE LEAVES OF PLANTS FROM HITTING THE GROUND.

DRAINAGE BASINS

• A RIVER’S DRAINAGE BASIN (OR CATCHMENT AREA) IS THE AREA DRAINED BY A RIVER AND ITS TRIBUTARIES.

DRAINAGE DENSITY

• SOME AREAS HAVE A TIGHTLY PACKED NETWORK OF RIVERS, THAT MEANS IT HAS A HIGH DRAINAGE DENSITY. OTHER AREAS HAVE FEW RIVERS, THAT MEANS THEY HAVE A LOW DRAINAGE DENSITY.

WHAT INFLUENCES DRAINAGE DENSITY?

• THERE ARE MANY RIVERS WHERE:– RAINFALL IS HIGH– THE ROCKS ARE IMPERMEABLE AND

CANNOT ABORB WATER VERY QUICKLY

• THERE ARE FEW RIVERS WHERE:– RAINFALL OS LOW– THERE ARE PERMEABLE ROCKS, WHICH

CANNOT ABSORB RAINFALL QUICKLY

A RIVER’S DRAINAGE BASIN

DRAINAGE PATTERNS

• RIVERS OFTEN FORM A DISTINCT PATTERN WHICH IS DUE TO THE STRUCTURE OF THE ROCKS IN THE DRAINAGE BASIN.

• THERE ARE 3 TYPES OF DRAINAGE PATTERNS RIVERS FOLLOW – 1.DENDRITIC2.RADIAL3.TRELLISED

DENDRITIC PATTERN

• THIS DRAINAGE PATTERN RESEMBLES THE BRANCHES OF A TREE. THE MAIN RIVER IS THE “TRUNK” AND THE OTHER TRIBUTARIES ARE THE “BRANCHES”.THIS PATTERN USUALLY DEVELOPS IN GENTLY SLOPING AREAS

RADIAL PATTERN

• THIS DRAINAGE PATTERN RESEMBLES THE SPOKES OF A WHEEL. THE RIVERS FLOW OUTWARDS IN ALL DIRECTIONS FROM A CENTRAL POINT. RIVERS ON VOLCANIC CONES USUALLY DEVELOP THIS PATTERN.

TRELLSIED PATTERN

• THIS DRAINAGE PATTERN RESEMBLES A RECTANGULAR GRID. THIS PATTERN USUALLY DEVELOPS IN AREAS WHER THERE ARE DIFFERENT TYPES OF ROCK (SOME ARE PERMEABLE AND SOME ARE IMPERMEABLE). THE MAIN RIVERS RUN NORTH OR SOUTH AND THE TRIBUTARIES RUN EAST OR WEST(AT RIGHT ANGLES TO THE MAIN RIVER).

RIVER PROCESSES

• EROSION– HYDRAULIC ACTION: THIS IS THE FORCE

OF THE RIVER ITSELF REMOVING LOOSE MATERIAL FROM THE BED AND BANKS OF THE RIVER

– CORRASION: (OR ABRASION) THIS IS THE PROCESS BY WHICH STONES, SAND GRAINS AND OTHER PARTICLES ARE THROWN AGAINST THE BED AND BANKS TO ERODE THEM

– ATTRITION:STONES AND ROCKS CARRIED DOWNSTREAM ARE THROWN AGAINST EACH OTHER. THEY ARE WORN AWAY AND OVER TIME BECOME SMALLER.

– SOLUTION: RIVER WATER CAN DISSOLVE SOME OF THE MINERALS IN THE ROCKS, ESPECIALLY LIMESTONE

• TRANSPORT– SOLUTION: SOME MATERIALS ARE

DISSOLVED IN THE WATER– SUSPENSION: FINE CLAY PARTICLES

FLOAT ALONG IN THE RIVER WATER– TRACTION: STONES AND BOULDERS ARE

ROLLED ALONG THE RIVERBED.– SALTATION: SAND PARTICLES ARE

BOUNCED ALONG THE RIVERBED.

• DEPOSITION

• IF A RIVER DOES NOT HAVE ENOUGH ENERGY AVAILABLE, IT WILL NOT DEPOSIT ITS ENTIRE LOAD AT ONCE. THE HEAVIEST MATERIAL IS ALWAYS DEPOSITED FIRST.

• LARGE BOULDERS ARE DEPOSITED FIRST. THEY CAN ONLY BE MOVED WHEN THE SLOPE IS STEEP AND THE RIVER IS FULL OF WATER

• GRAVEL, SMALL STONES AND SAND ARE DEPOSITED NEXT.

• FINE CLAY PARTICLES SETTLE VERY SLOWLY

• SILT IS CARRIED ALONG WHEN THE WATER IS MOVING, AND IS DEPOSITED AFTER A FLOOD OR AT THE MOUTH OF THE RIVER.

• THE DISSOLVED MATERIAL IS NOT USUALLY DEPSOSITED, BUT IS CARRIED INTO THE SEA WITH THE RIVER WATER.

STAGES OF A RIVER

• THE UPPER STAGE

• THE MIDDLE STAGE

• THE LOWER STAGE

THE MAIN FEATURES OF THE UPPER STAGE ARE:

• A STEEP V-SHAPED VALLEY

• A STEEP GRADIENT

• INTERLOCKING SPURS

• POTHOLES

• WATERFALLS, RAPIDS AND GORGES

V-SHAPED VALLEYS

• IN THE UPPER STAGE MUCH OF THE RIVER’S ENERGY IS NEEDED TO TRANSPORT LOAD. THERE ARE LARGE BOULDERS WHICH CAUSE RAPID EROSION. THIS RESULTS IN THE RIVER CUTTING DOWNWARDS IN ITS BED CAUSING VERTICAL EROSION AND FORMS A V-SHAPED VALLEY.

INTERLOCKING SPURS

• THE RIVER WINDS ITS WAY (MEANDERS) AROUND OBSTACLES OF HARD ROCK. THIS EVEVTUALLY CREATES SPURS WHICH FORM ON EACH SIDE OF THE RIVER – SO THEY INTERLOCK.

POTHOLES

• WHERE THE RIVER BED IS UNEVEN, PEBBLES CARRIED BY FAST, SWIRLING WATER, ROTATE IN A CIRCULAR MOVEMENT AND ERODE THE BED FORMING POTHOLES. (JUST LIKE THE POTHOLES IN THE ROADS!)

WATERFALLS, GORGES AND RAPIDS

• THESE ARE FORMED BY DIFFERENCES IN THE TYPE OF ROCK, AND THE SOFTER ROCK ERODES MORE QUICKLY THAN THE HARDER ROCK … as shown in the following diagram

THE MIDDLE STAGE

• IN THIS STAGE THE RIVER HAS MANY TRIBUTARIES AND HAS MORE WATER. THIS MEANS IT HAS MORE ENERGY AND CARRIES OUT MORE EROSION. THE RIVER IS WIDENED BY LATERAL EROSION AND HAS THE FOLLOWING FEATURES:– A WIDER V-SHAPED VALEY– MEANDERS

MEANDERS

• A MEANDER IN GENERAL IS A BEND IN THE WATERCOURSE. A MEANDER IS FORMED WHEN THE MOVING WATER IN A STREAM ERODES THE BANKS AND WIDENS ITS VALLEY.

THE LOWER STAGE

• THE RIVER CONTIUES TO ERODE DOWNWARDS AND THE VALLEY IS CLOSE TO SEA LEVEL. THE RIVER BECOMES WEAKER BECAUSE THERE IS LESS WATER WHICH MEANS LESS ENERGY. THEREFORE, SEDIMENT IS DEPOSITED.

FEATURES OF THE LOWER STAGE ARE:

• A VERY WIDE VALLEY• MANY MEANDERS• FREQUENT CHANGES IN THE COURSES OF

THE RIVER• A RIVERBED OF SILT AND CLAY – CALLED

FLOOD PLAINS• OXBOW LAKES• LEVEES• DELTAS

OX-BOW LAKES

• An oxbow lake is a U-shaped body of water formed when a wide meander from the main stem of a river is cut off to create a lake.

• An oxbow lake is formed when a river creates a meander, due to the river's eroding the bank through hydraulic action and abrasion/corrosion. After a long period of time, the meander becomes very curved, and eventually the neck of the meander will touch the opposite side and the river will cut through the neck, cutting off the meander to form the oxbow lake.

FLOOD PLAIN

• AS A RIVER FLOWS FROM ITS MIDDLE COURSE TO ITS LOWER COURSE, MEANDERS BECOME EVEN MORE PRONOUNCED, AND THE VALLEY BECOMES WIDER AND FLATTER. WHEN SILT IS DEPOSITED ON THE VALLEY FLOOR MAINLY WHEN THE RIVER FLOODS, THIS RESULTS IN THE FLOOD PLAIN OF THE RIVER.

LEVEES

• THE RIVER BANK MAY SOMETIMES BE HIGHER THAN THE FLOOD PLAIN. A LEVEE IS A RAISED RIVER BANK BUILT UP FROM ALLUVIAL (SILT) DEPOSITS

• HOW ARE THEY FORMED?– AFTER A PERIOD OF RAIN THE RIVER FLOODS

AND FLOWS OVER– THE FLOOD CAUSE SILT TO BE DEPOSITED– WHEN THE FLOOD WATERS RECEDE, A THICK

DEPOSIT OF MUD IS LEFT AT THE RIVER’S BANKS. AFTER THIS PROCESS HAS BEEN REPEATED A LEVEE IS FORMED.

DELTAS

• DELTAS ARE FORMED BY THE DEPOSITION OF SEDIMENT AT THE MOUTH OF A RIVER AS IT ENTERS A SEA OR LAKE. DELTAS ONLY FORM UNDER CERTAIN CONDITIONS AND MOST RIVERS DO NOT END IN A DELTA.