THE DRAINAGE BASIN

 

THE DRAINAGE BASIN IS AN AREA PROVIDING WATER RUNOFF AND SEDIMENT SUPPLY TO, AND SUSTAINING PART OR ALL OF THE FLOW OF, A STREAM  CHANNEL.

  

 

IT IS ALSO A VERY IMPORTANT UNIT FOR STUDYING THE LANDSCAPE

 

BECAUSE IT IS OF LIMITED EXTENT AND HAS A GENERALLY CLEARLY DEFINED PERIMETER CALLED A DRAINAGE DIVIDE (OR WATERSHED) RUNNING THROUGH HIGHER TOPOGRAPHY. IT IS RELATIVELY EASY TO MONITOR THE INPUTS (PPT), STORAGE AND OUTPUTS OF WATER AND SEDIMENT AND SOLUTES IN SUCH A SYSTEM.

 

IT IS BOTH A CASCADING SYSTEM AND A PROCESS-RESPONSE SYSTEM

 

WHICH RESPONDS TO VARIATIONS OF THE ENVIRONMENTAL CONTROLS AND THEREFORE OF PROCESSES BY EROSION OR AGGRADATION.

 

THE FORM OF THE DRAINAGE BASIN

 

IS A PRODUCT OF INHERITANCE - OF THE EFFECTIVENESS OF PAST PROCESSES ON THE SOLID GEOLOGY, E.G. THE IMPACT OF GLACIATION DURING THE PLEISTOCENE AND THE IMPORTANCE OF GLACIAL OR PARAGLACIAL MATERIAL,

OR OF WEATHERING AND FLUVIAL EROSION DURING THE TERTIARY PERIOD. IN THE TROPICS THIS HAS CONTINUED UNINTERRUPTED FOR MILLIONS OF YEARS.

 

CHANGES IN THE RELATIVE IMPORTANCE OF THE PROCESSES

 

HAVE OCCURRED OVER TIME, E.G. GLACIATION, SLOPE PROCESSES, AGGRADATION AND EROSION BY STREAMS.

 

THE FORM OF THE BASIN INFLUENCES THE CONTEMPORARY PROCESSES.

 

 

 

THE CHARACTER OF THE DRAINAGE BASIN IS SHAPED BY THE FOLLOWING:

 

1) CLIMATE

 

2) GEOLOGY

 

3) THE RELIEF OF:

 

A) THE BASIN

 

B) THE STREAM CHANNEL  

(SLOPE, DRAINAGE DENSITY ETC., 1-5% OF THE DRAINAGE BASIN AREA IS OCCUPIED BY STREAM CHANNELS)

 

DRAINAGE BASINS

 

ARE GEOMORPH-OLOGICAL SYSTEMS IN WHICH IT IS POSSIBLE TO STUDY THE RELATIONSHIPS BETWEEN LANDFORMS AND PROCESSES.

 

SCALE 

 

 THEY OCCUR AT ALL SCALES FROM RILLS WITH DRAINAGE BASINS OF 2-3M2 TO THE AMAZON BASIN (6.3 MILLION KM2.)

 

 

INPUTS AND OUTPUTS OF WATER AND SEDIMENT:

 

1)    THE WATER BALANCE &

 

 

      2)  THE SEDIMENT BUDGET

 

BOTH USE A CONTINUITY EQUATION APPROACH TO MEASURE  THE  INPUTS, THROUGHPUTS, OUTPUTS AND STORAGE OF WATER AND SEDIMENT.

IT IS POSSIBLE TO QUANTIFY THE INPUTS AND OUTPUTS.

 

1) THE WATER BALANCE

 

DS = P - (ET + R )

 

WHERE DS IS THE CHANGE IN STORAGE PER UNIT TIME, P = PPT,

ET = EVAPOTRANSPIRATION AND R = RUNOFF

 

WE CAN MEASURE PPT DIRECTLY MM/UNIT TIME.

 

WE CAN COMPUTE RUNOFF IF WE MEASURE Q AND KNOW BASIN SIZE. RUNOFF IS MEASURED AS A DEPTH EQUIVALENT=MM/UNIT TIME.  

 

 EVAPOTRANSPIRATION IS DIFFICULT TO MEASURE SO IT IS USUALLY FOUND AS A RESIDUAL. THIS IS EXPRESSED IN MM/UNIT TIME.

 

OVER THE LONG TERM (>1 YR) CHANGES IN STORAGE TEND TO = 0. THERE ARE YEARS OF INCREASED AND DECREASED STORAGE AND THESE EQUALIZE EACH OTHER OUT. SO WE CAN REWRITE THE EQUATION AS

 

             P = ET + R

 

        OR        P- R = ET

 

EG.  THE HUMBER BASIN

PPT = 751 MM/YR

R = 434 MM/YR     

751 MM/YR - 434 MM/YR = 317 MM/YR       EVAPOTRANSPIRATION

 

PPT EITHER SITS ON THE SURFACE AND EVAPORATES, RUNS OFF AND OR INFILTRATES INTO THE SOIL (ENTERS STORAGE).

 

WHICH OF THESE OCCURS DEPENDS ON THE CHARACTER OF THE DRAINAGE BASIN SURFACE - ESPECIALLY ITS PERMEABILITY

 

PERMEABILITY IS INFLUENCED BY FACTORS SUCH AS –

 

1) SOLID GEOLOGY

2) REGOLITH - WEATHERED OR TRANSPORTED MATERIAL

3) SOIL TYPE

4) VEGETATION COVER &

5) LAND USE.

 

IMPERMEABLE SURFACES/SOILS CAUSE MORE RUNOFF. THIS ERODES THE SURFACE. SUCH BASINS  ARE CHARACTERISED BY A HIGHER DRAINAGE DENSITY.

 

 MORE PERMEABLE SURFACES/SOILS ALLOW MORE INFILTRATION AND THEREFORE LESS RUNOFF AND LESS SOIL EROSION. SUCH BASINS ARE CHARACTERISED BY A LOW DRAINAGE DENSITY.

 

THE CHARACTER OF THE SURFACE INFLUENCES THE ROUTING OF THE WATER AND SEDIMENT TO THE CHANNEL. RUNOFF SUPPLIES STORM- GENERATED FLOW

(STORMFLOW) AND PARTICULATES (SUSPENDED LOAD) TO THE CHANNEL.  

 

MORE INFILTRATION MEANS LESS RUNOFF. INFILTRATION IS RESPONSIBLE FOR SOLUTION WEATHERING AND SUBSURFACE EROSION, FOR PUTTING WATER INTO STORAGE AND ALSO FOR GROUNDWATER FLOW (BASEFLOW) AND ITS CONTAINED DISSOLVED SOLIDS (SOLUTION LOAD) TO THE CHANNEL.

 

THE INFILTRATION RATE IS CALCULATED BY:

 

          Ir = d  = DEPTH OF WATER OF WATER RECEIVED (MM)

                 t  = UNIT TIME ( HR)

 

      IF RAINFALL INTENSITY ( MM/HR) > INFILTRATION RATE (MM/HR) SHEETFLOW OR WASH - HORTONIAN OVERLAND FLOW - WILL OCCUR:

 

HOLF/t = Ri/t -Ir/t

 

THE OVERLAND FLOW CAN BECOME CONCENTRATED BY SURFACE IR-REGULARITIES TO FORM RILLS AND GULLIES (CHANNELS). H.O.L.F. IS A VERY RARE PROCESS IN THE TEMPERATE ZONE WHERE THERE IS A LOT OF VEGETATION AND INTENSE RAINFALL IS RARE.

 

SATURATION OVERLAND FLOW OCCURS WHEN THE VOIDS IN THE SOIL ARE FILLED WITH WATER AND NO MORE WATER CAN ENTER. THIS MAY OCCUR AFTER A LONG PERIOD OF RAIN BUT AGAIN IT IS QUITE RARE.

 

INFILTRATION RATES ARE INFLUENCED BY SOIL TEXTURE AND VEGETATION COVER. E.G. 

YORK CAMPUS:

BARE SOIL : 

WET CLAYS         0 - 4 MM/HR

WET SANDS         3 - 12 MM/HR                                

WET SANDS UNDER VEG. (OLD PASTURE)  66MM/HR

 

THE RATE OF GROUNDWATER FLOW (BASEFLOW) DEPENDS ON THE PERMEABILITY AND POROSITY OF THE SOIL/ROCK. GROUNDWATER FLOW IS PROBABLY MOST IMPORTANT IN HUMID TROPICAL AND HUMID TEMPERATE ENVIRONMENTS.

 

THERE ARE BASINS IN SOUTHERN ONTARIO THAT ARE DOMINATED BY RUNOFF OR GROUNDWATER FLOW -

DEPENDING ON THE ROCK TYPE AND THE CHARACTER OF THE SURFACE.

 

GROUNDWATER FLOWS VERY SLOWLY COMPARED TO SURFACE WATER FLOW, E.G. FINE POROUS ROCKS < 1MM/DAY, IN WELL-JOINTED LIMESTONES 5500M/DAY.

 

THE RUNOFF-INFILTRATION MODEL IS THE BASIS FOR SEPARATING HYDROGRAPHS INTO TWO COMPONENTS.

 

A) STORMFLOW/SURFACE RUNOFF

 

AND 

B) BASEFLOW/ GOUNDWATER FLOW

 

THE TWO COMPONENTS OF STREAMFLOW ARE

 MADE UP OF:

 

1) STORMFLOW (RUNOFF) WHICH OCCURS DURING OR RELATIVELY SOON AFTER A STORM. PULSES OF WATER FLOW DOWN THE SLOPES AND IS SUPPLIED BY THE FOLLOWING:

 

A) CHANNEL PRECIPITATION

 

 (PPT DIRECTLY IN TO THE CHANNEL) THIS IS INSIGNIFICANT BECAUSE CHANNELS REPRESENT < 5% OF DRAINAGE BASIN AREA. THERE IS A  RAPID CONVERSION TO Q (DISCHARGE).

 

B) OVERLAND FLOW/STORMFLOW

    

 A) HORTONIAN OVERLAND FLOW

                                           

 B) SATURATION OVERLAND FLOW

 

RATE OF FLOW:

 

200- 400 M/HR DURING OR SHORTLY AFTER RAINFALL AND DURING SNOWMELT.

 

AND

 

C)  SOME THROUGHFLOW

(SUBSURFACE STORMFLOW - COMBINATION OF THROUGHFLOW AND ACCELERATED GROUNDWATER FLOW) OCCURS IN THE GROUND ABOVE THE GROUNDWATER TABLE. VERY SLOW RATES - < 0.5 M/HR TO < 0.5 M/WK.

CAN BECOME PART OF EITHER STORMFLOW OR BASEFLOW.

 

 

 

2) BASEFLOW

     (GROUNDWATER) SUBSURFACE FLOW THAT IS SLOWLY RELEASED OVER A LONG PERIOD

 

 

A) BASEFLOW  FLOW OCCURING BELOW THE GROUNDWATER TABLE. VERY SLOW, SEVERAL WEEKS, MONTHS TO 10,000 YR LAG BETWEEN RAINFALL AND Q.

 

B) SOME THROUGHFLOW

 

 

THESE COMPONENTS VARY IN IMPORTANCE

 

FROM ONE DRAINAGE BASIN TO ANOTHER, AND ALSO SEASONALLY. BASINS IN SOUTHERN ONTARIO RECEIVE BETWEEN 0 AND 90% OF THEIR Q FROM BASEFLOW.

 

WHERE INFILTRATION IS HIGH FLOOD PEAKS ARE LOW AND THE BASIN IS SAID TO HAVE A “BASEFLOW REGIME”.

 

WHERE INFILTRATION IS LOW, THE FLOOD PEAKS ARE HIGH, BASEFLOW IS LOW. THE REGIME IS IRREGULAR AND THE BASIN IS SAID TO HAVE AN EPHEMERAL OR “FLASHY” REGIME.

 

DRAINAGE BASINS ARE DYNAMIC. THEY RESPOND DIFFERENTLY TO DIFFERENT STORMS, ETC. BECAUSE OF THE STORM SIZE, (WHICH INFLUENCES THE AREA OF THE DRAINAGE BASIN CONTRIBUTING TO RUNOFF ETC.- VARIABLE SOURCE CONCEPT. RAINFALL INTENSITY, DIFFERENT TEMPERATURES - BECAUSE THEY EFFECT FREEZING OF THE SOIL AND PERMEABILITY AND HOW THE PPT IS DELIVERED ARE OTHER IMPORTANT VARIABLES.

 

 

THE AMOUNT OF WATER CONTRIBUTED BY THE 4 ROUTES MENTIONED ABOVE DEPENDS ON 2 SETS OF FACTORS RELATED TO:

 

1) DRAINAGE BASIN CHARACTERISTICS     SOIL TYPE

 

                                                                            VEGETATION

 

                                                                             NATURE OF THE CHANNELS

 

                                                           LANDUSE- AGRICULTURE, URBANIZATION

 

AND TRANSITORY CHARACTERISTICS SUCH AS SOIL MOISTURE AND THE POSITION OF THE WATERTABLE.

 

AND

 

 2) THE PRECIPITATION EVENT    

 

INTENSITY

 

AREA COVERED/SIZE

 

DURATION

 

SOME DRAINAGE BASIN CHARACTERISTICS ARE FAIRLY PERMANENT AND CHANGE VERY SLOWLY, E.G. SOIL TYPE.

 

2) THE SEDIMENT BUDGET

 

IS A QUANTITIVE STATEMENT OF THE RATES OF PRODUCTION, MOBILIZATION AND DISCHARGE OF SEDIMENT FROM A BASIN.

 

TO CONSTRUCT A SEDIMENT BUDGET WE HAVE TO RECOGNIZE AND QUANTIFY THE RATE AND VOLUME OF TRANSPORT PROCESSES AND THE SITES OF TEMPORARY SEDIMENT STORAGE (EG. BASE OF SLOPES, FLOODPLAINS ETC.)

 

THE RATES AND VOLUMES OF MATERIAL MOVED BY THE TRANSPORT PROCESSES CHANGE OVER TIME, E.G. THE INFLUENCE OF LARGE STORMS AND PEAK DISCHARGES MAY CAUSE MORE SLOPE INSTABILITY AND MORE CHANNEL CHANGES THAN THE TYPES OF PROCESSES SEEN IN THE INTERVENING PERIODS-CREEP, SMALL-SCALE BANK EROSION.

 

IN THE INTERVENING PERIODS MOST SEDIMENT RETURNS TO STORAGE ON THE LOWER PARTS OF SLOPES AND IN THE BED AND BANKS OF GULLIES AND CHANNELS AND SMALL AMOUNTS OF SEDIMENT ARE MOVED SHORT DISTANCES AFTER BEING MOBILIZED BY RAINSPLASH, WASH OR LOCAL SLIDING.

E.G. OF

 

 

 

 

SEDIMENT BUDGET FOR

THE UPPER VAN DUZEN BASIN (525KM2) 1941-1975

 

 

BUDGET COMPONENT                 MASS OF SEDT(t)      % TOTAL INPUT

 

 

INPUT

 

 

FLUVIAL & SEDIMENT YIELD FROM

HILLSLOPES                                                         45,509,000                           73

 

 

LANDSLIDING INTO MAIN CHANNEL:

    DEBRIS SLIDES, DEBRIS AVALANCHES     10,630,000                           17

    EARTHFLOWS                                                    2,931,000                            5

 

STREAMBANK EROSION, MAIN CHANNEL :

   MELANGE BANK EROSION                                  426,000                            1

   FLOODPLAIN & FILL TERRACE EROSION      2,619,000                            4

                                                          

                                                            TOTAL        62,115,000

 

STORAGE

 

AGGRADATION IN MAIN CHANNEL                 10,601,000

 

 

OUTPUT

TOTAL SEDIMENT DISCHARGE

 FROM BASIN                                                        51,036,000

 

 

 

 

 

 

MAJOR CHANGES IN THIS BASIN AND IN OTHERS ARE BROUGHT ABOUT BY EXTREMELY HIGH MAGNITUDE EVENTS. IN THE UPPER VAN DUZEN BASIN THIS WAS A MAJOR STORM AND FLOOD IN DEC 1964 WHICH LASTED FOR 3 DAYS AND ACCOUNTED FOR 7% OF SEDIMENT TRANSPORT IN THE BUDGET. THE STORM/FLOOD MOBILIZED AS MUCH SEDIMENT AS  IS MOVED OUT OF THE BASIN OVER A 100YEAR PERIOD.

 

THIS TYPE OF RELATIONSHIP BETWEEN LESS EXTREME EVENTS AND THE SEDIMENT BUDGET CAN BE SEEN IN OTHER BASINS, E.G. SPRING SNOW-MELT WHERE FLOODING IS ASSOCIATED WITH OVER 80% OF THE ANNUAL SUSPENDED SEDIMENT LOAD OF THE FRASER, IN THE ASSINIBOINE SNOW-MELT FLOODING IN APRIL-MAY ACCOUNTS FOR 70% OF LOAD, AND SEDIMENT IS COMPOSED OF ALMOST 40% SAND. FOR THE REST OF THE YEAR THE LOAD >85% CLAY & SILT.

 

 IN THE GREAT LAKES BASINS SUSPENDED SEDIMENT CONCENTRATION  HAS AN ANNUAL PATTERN. HIGH IN THE SPRING BECAUSE OF SNOW-MELT FLOODING, LOW DURING THE SUMMER AND HIGH AGAIN IN THE FALL BECAUSE OF INCREASING PPT.

 

THE AMOUNT OF SURFACE LOWERING IN A DRAINAGE BASIN HAS BEEN STUDIED. IN CORDILLERAN BASINS IN SOUTHERN ALBERTA IT WAS FOUND THAT DENUDATION AMOUNTED TO 60 M3/KM2/YR (= 60 BUBNOVS ) OR 3840 TONNES/KM2/YEAR. OF THIS 30 BUBNOVS LEFT THE BASIN AS DISSOLVED. SOLIDS AND 30 BUBNOVS AS SUSPENDED LOAD.IN THE HUMBER BASIN IN S. ONTARIO THERE IS APPROX.1.2M3/KM2/YR LOWERING OR 94 METRIC TONNES/KM2 /YR.

 

BASINS RESPOND TO CHANGES IN THE FUNDAMENTAL CONTROLS (CLIMATE, GEOLOGY, BASE LEVEL) AND TO THE ACTIVITIES OF HUMANS BY ERODING OR AGGRADING.

 

THERE ARE MANY INSTANCES WHERE CLIMATICALLY-INDUCED FLUCTUATIONS IN THE RATE AND DOMINANCE OF DIFFERENT PROCESSES HAVE LEAD DIFFERENT TYPES OF DEPOSITS, E.G. TERRACE GRAVELS AND COVERLOAMS AND TO CYCLICAL CHANGES IN LANDSCAPE EVOLUTION AS DEMONSTRATED IN THE GEOMORPHOLOGICAL AND SEDIMENTLOGICAL RECORDS. SUCH EPISODES (CAUSED BY TECTONISM OR ISOSTASY/EUSTASY) CONSIST OF

 

1)    INCISION OR DOWNCUTTING

2)    LATERAL EROSION & PEDIPLANATION

      3)  DEPOSITION (AGGRADATION) AND

      4)  STABILITY ASSOCIATED WITH SOIL DEVELOPMENT

 

SUCH CYCLES ARE CALLED EDS CYCLES = E (ROSION) D (EPOSITION) AND S (TABILITY) CYCLES. THEIR DURATION AND FREQUENCY RANGES OVER SEVERAL ORDERS OF MAGNITUDE AND ARE RELATED TO DIFFERENT GEOMORPHOLOGICALO THRESHOLDS:

 

MICROCYCLES   10 TO 100 YR MAGNITUDE.

MESOCYCLES      1000 TO 10,000 YR MAGNITUDE.

MACROCYCLES    95-125 KA. MAGNITUDE.

MEGACYCLES      400-500KA MAGNITUDE.

 

IN CANADA AND OTHER AREAS AFFECTED BY GLACIATION THE DRAINAGE SYTEMS HAVE BEEN DERANGED.

 

THE DRAINAGE BASINS MODIFIED BY GLACIAL PROCESSES AND DEPOSITION. THE RIVERS ARE GEOLOGICALLY YOUNG AND THERE ARE NO TRULY INTEGRATED RIVER SYSTEMS THAT APPROACH THE THEORETICALLY IDEAL DENDRITIC NETWORK OF TEXTBOOKS.

 

THE STREAM GRADIENTS HAVE BEEN ALTERED BY GLACIATION AND DRAINAGE DIVIDES HAVE BEEN BREACHED.

 

THERE ARE NUMEROUS LAKES THAT CAPTURE SEDIMENT. THE ST. LAWRENCE RIVER, FOR EXAMPLE IS 3058KM LONG, HAS A LARGE BASIN  BUT CARRIES VERY LITTLE SEDIMENT.

 

TEXTBOOKS TELL US THAT THERE IS A FINING OF SEDIMENT IN A DOWNSTREAM DIRECTION BUT IN MANY CASES IN CANADA THIS HAS BEEN COMPLICATED BY AN INFLUX OF GLACIAL OR PARAGLACIAL MATERIAL, SOME OF WHICH HAS BEEN CARRIED IN FROM OTHER DRAINAGE BASINS, SOME MATERIAL IS VERY FAR-TRAVELLED.

 

MANY PREGLACIAL VALLEYS HAVE BEEN FILLED OR PARTIALLY FILLED BY GLACIAL MATERIALS. SOME ARE NOW BEING RE-EXCAVATED BY CONTEMPORARY STREAMS, E.G. THE HUMBER, CREDIT AND DON RIVERS.

MANY STREAMS FLOW THROUGH PROGLACIAL SPILLWAYS.

 

 

A STREAM AND ITS BASIN CAN BE CLASSIFIED IN TERMS OF ITS RELATIVE POSITION WITHIN A DRAINAGE NETWORK OR HIERARCHY (MORPHOMETRY). MANY DIFFERENT SCHEMES HAVE BEEN SUGGESTED TO RANK STREAMS/DRAINAGE BASINS IN TERMS OF THEIR ORDER. THE SIMPLEST IS STRAHLER’S SYSTEM.

 

SEE OVERHEAD

 

 

 

 

 

 

NOTE THE PROGRESSIVE DECLINE IN THE NUMBER OF STREAMS AS ORDER INCREASES. THIS IS VERY USEFUL WHEN COMPARING STREAMS BECAUSE IT IS POSSIBLE TO COMPARE STREAMS OR DRAINAGE BASINS OF THE SAME ORDER.

 

THE DRAINAGE SYSTEM FORMED BY THESE STREAMS OF VARIOUS ORDER IS CALLED A DRAINAGE NETWORK.

 

THE GEOMETRIC PATTERN OR SHAPE OF A NETWORK DOES NOT SEEM TO BE RELATED TO STREAM ORDER.

 

CERTAIN LINEAR PARAMETERS OF A BASIN ARE PROPORTIONALLY RELATED TO THE STREAM ORDER AND COULD BE EXPRESSED AS BASIC RELATIONSHIPS OF THE DRAINAGE  SYSTEM, E.G. BIFURCATION RATIO, LENGTH RATIO

 

ONE CHARACTERISTIC OF A DRAINAGE BASIN OR DRAINAGE NETWORK IS ITS DRAINAGE DENSITY.

 

THE DRAINAGE DENSITY IS GIVEN BY THE UNIT LENGTH OF STREAM CHANNEL

(KM) PER UNIT AREA OF DRAINAGE BASIN (KM2). BASINS WITH HIGH DRAINAGE DENSITY HAVE MANY INTERFINGERING CHANNELS. THEY HAVE IMPERMEABLE SURFACES. THEY HAVE SHORT DISTANCES OF OVERLAND FLOW,  THEREFORE  WATER GETS TO THE STREAM QUICKLY AND IS SOON TURNED IN TO Q. THERE IS A MORE-DIRECT RESPONSE TO PRECIPITATION.

 

BASINS WITH PERMEABLE SURFACES HAVE LOW DRAINAGE DENSITY. THEY HAVE FEW CHANNELS, BECAUSE MOST WATER ROUTING OCCURS VIA BASEFLOW. THERE IS A LONG DISTANCE OF OVERLAND FLOW & THEREFORE THE PPT. TAKES MUCH LONGER TO GET TO THE CHANNEL (GREATER LAG TIME ).

 

E.G. OF DRAINAGE DENSITY - THE DON RIVER BASIN

 

BASIN AREA       370 KM2

TOTAL CHANNEL LENGTH 334 KM.

 

  DU = S L (TOTAL LENGTH OF CHANNELS)

            Ad

 

 DU = 334    = 0.9 KM OF CHANNEL PER KM2 OF BASIN

          370

 

      < 5 SO IT CAN BE DEFINED AS A COARSE DRAINAGE DENSITY TYPICAL OF THE HUMID TEMPERATE AREAS OF NORTH AMERICA.

 

A MORE TELLING MEASURE IS THE INVERSE RELATIONSHIP.  THE MINIMUM UNIT AREA OF BASIN NECESSARY TO SUPPLY WATER AND SEDIMENT TO MAINTAIN A UNIT LENGTH OF STREAM CHANNEL.

THIS IS CALLED THE CONSTANT OF CHANNEL MAINTENANCE.

 

E.G. THE DON RIVER BASIN

 

                C = Ad

                       SL

 

                C = 370   = 1.10 KM2/KM CHANNEL

                      334

 

THERE IS A NEED FOR A GREATER AREA OF BASIN TO SUPPLY THE CHANNEL WITH WATER AND DEBRIS IN MORE ARID AREAS (& IN RAINFORESTS WERE A LOT OF THE PPT IS EVAPORATED OR EVAPOTRANSPIRED BEFORE REACHING THE GROUND/STREAM.  BECAUSE OF A LACK OF PPT AND GREATER EVAPO-TRANSPIRATION RESULTING IN LESS OVERLAND FLOW. EG.IN ARIZONA C = 316 KM2/KM.

 

Cf.

ALBERNI B.C.        Du  0.9       CONSTANT OF CH M. 1.11

SIBBESTON LAKE N.W.T.      Du 0.75  CCM 1.3

PEACE POINT AL                    Du 0.71 CCM 1.4

KITCHENER ON                      Du 0.5 CCM 1.9