Running water and Groundwater
Water Reservoirs
•97.2% stored in the oceans as saltwater
•2.15% exists in solid phase as glaciers, ice sheets and icebergs
•0.65% divided amongst freshwater, atmosphere
Earth as a system: the hydrologic cycle
•Illustrates the circulation of Earth’s water supply
•Processes involved in the cycle
–Precipitation
–Evaporation
–Infiltration
–Runoff
–Transpiration
Annual Water Balance
•Atmosphere:
–Input: 320,000 + 60,000 km3 (evapotranspiration)
–Output: 284,000 + 96,000 km3 (precipitation)
•Oceans:
–Input: 284,000 + 36,000 km3 (precipitation, runoff)
–Output: 320,000 km3 (evaporation)
•Land:
–Input: 96,000 km3 (precipitation)
–Output: 60,000 + 36,000 km3 (evapotranspiration, runoff/infiltration)
(Im)balance
•The hydrologic cycle is balanced because no water leaves the system
•Imbalances can exist temporarily
–If inputs are less than outputs, a deficit exists
–If outputs are less than outputs, a surplus exists
Runoff / Infiltration
•Water leaves land by runoff / infiltration
•Relative amounts determined by:
–Intensity/duration of rainfall
–Existing soil moisture
–Surface material
–Slope
–Vegetation
Runoff
•Drainage basin
–Land area that contributes water to a river system
–A divide separates drainage basins
Sediment
•Running water is the dominant sediment source
–Sediment production occurs via erosion in the headwater portion of river system (high velocity)
–Transport occurs in the river trunk (medium velocity)
–Deposition occurs as river debouches into a receiving basin (low velocity)
Streamflow
•Running water may move in two ways
–Laminar Flow
•Particles move in straight lines
•Exhibits smooth surface
–Turbulent Flow
•Particles move in erratic trajectories
•Exhibit eddies, whirlpools, rapids
Running water
•Streamflow
–Factors that determine velocity
•Gradient, or slope
•Channel characteristics
–Shape
–Size
–Roughness
•Discharge – volume of water flowing in the stream (generally expresses as cubic feet
per second)
•Upstream-downstream changes
–Profile
•Cross-sectional view of a stream
•From head (source) to mouth
–Profile is a smooth curve
–Gradient decreases from the head to the mouth
•Factors that increase downstream
–Velocity (units: metres/second)
–Discharge (units: metres3/second)
»Also, cross-sectional area times velocity
•Factors that increase downstream
–Channel size
•Factors that decrease downstream
–Gradient, or slope
–Channel roughness
•The work of streams
–Erosion
–Transportation
•Transported material is called the stream’s load
–Dissolved load
–Suspended load
–Bed load
Dissolved Load
•Usually small amounts
•Expressed in parts per million (ppm)
•Related to water chemistry only
•Typical river has a dissolved load of about 120 ppm
Suspended Load
•Transported sediment is suspended in water
•Responsible for muddy appearance
•Capacity and particle size increase with discharge
Bed Load
•Large particles too heavy to be suspended accumulate at stream bottom
•These particles move by rolling, sliding and saltation
•Important for erosion of stream bottom
•Usually represents about 10% of total stream load
Running water
•The work of streams
–Transportation
•Load is related to a stream’s
–Competence – maximum particle size
»Related to velocity2
–Capacity – maximum load
–Capacity is related to discharge
•Deposition
–Caused by a decrease in velocity
–Competence is reduced
–Sediment begins to drop out
•Stream sediments
–Known as alluvium
–Well-sorted deposits
Stream channels
•2 types of stream channel
–Bedrock Channel
•Steep slope, high velocity
•High erosional potential
–Alluvial Channel
•Unconsolidated sediment
•Shape affected by gradient, discharge, particle size
•2 forms: meandering and braided
Running water
•Base level
–Lowest point a stream can erode to
–Two general types
•Ultimate – sea level
•Temporary, or local
–Changing causes readjustment of the stream
– deposition or erosion
Base Level
•Resistant bedrock can act as temporary base level
•After faulting, stream changes where erosion and deposition occur
Running water
•Stream valleys
–Valley sides are shaped by
•Weathering
•Overland flow
•Mass wasting
–Characteristics of narrow valleys
•V-shaped
•Downcutting toward base level
Characteristics of narrow valleys
•Features often include
–Rapids
–Waterfalls
–Characteristics of wide valleys
•Stream is near base level
–Downward erosion is less dominant
–Stream energy is directed from side to side
–Characteristics of wide valleys
•Floodplain
•Features often include
–Meanders
–Cutoffs
–Oxbow lakes
•The work of streams
–Transportation
•Features produced by deposition
–Deltas – exist in ocean or lakes
–Natural levees – form parallel to the stream channel
–Area behind the levees may contain back swamps or yazoo tributaries
Deltas
•The bottom line: Deltas exist where the river carried sediment accumulates faster than marine
processes can remove it.
•Occur wherever a river debouches into oceans, bays, marginal seas, lakes
Running water
•Drainage patterns
–Networks of streams that from distinctive patterns
–Types of drainage patterns
•Dendritic
•Radial
•Rectangular
•Trellis
•Floods and flood control
–Floods are the most common geologic hazard
–Causes of floods
•Weather
•Human interference with the stream system
–Engineering efforts
•Artificial levees
•Flood-control dams
•Channelization
–Nonstructural approach through sound floodplain management
Groundwater
•Movement of water underground
–Mostly movement within pore spaces
–Rarely in underground rivers
–Important source for surface water
–Caused by gravity and pressure gradient
–All fluids move from high to low pressure
Freshwater Hydrosphere
Part of the Hydrosphere Volume
(km3)
•Ice sheets and glaciers
24,000,000
•Groundwater
4,000,000
•Lakes, reservoirs
155,000
•Soil Moisture
83,000
•Water Vapour
14,000
•River Water
1,200
Groundwater effects
•Caverns and sinkholes created by removal of soluble material
•Increased stress from human consumption
•Increased contamination from human activities
•Distribution and movement of groundwater
–Distribution of groundwater
•Zone of soil moisture
–Retained in O Horizon for plant use
•Zone of aeration
–Unsaturated zone
–Pore spaces in the material are filled mainly with air
•Zone of saturation
–All pore spaces in the material are filled with water
–Water within the pores is groundwater
•Water table – the upper limit of the zone of saturation
•Recharge – the downward infiltration of surface water that joins the water
table
•Porosity
–Percentage of pore spaces
–Determines storage of groundwater
•Permeability
–Ability to transmit water through connected pore spaces
–Aquitard – an impermeable layer of material
–Aquifer – a permeable layer of material
»Can be confined or unconfined
Measuring Movement
•Hydraulic gradient = (h1-h2)/d
•Q = KA(h1-h2)/d
–Q is discharge
–K is hydraulic conductivity
–A is cross-sectional area
•Features associated with groundwater
–Springs
•Hot springs
–Water is 6–9° C warmer than the mean air temperature of the locality
–Heated by cooling of igneous rock
•Geysers
–Intermittent hot springs
–Water turns to steam and erupts
–Wells
•Pumping can cause a drawdown (lowering) of the water table
•Pumping can form a cone of depression in the water table
–Artesian wells
•Water in the well rises higher than the initial groundwater level
•Environmental problems associated with groundwater
–Treating it as a nonrenewable resource
–Land subsidence caused by its withdrawal
–Contamination
•Geologic work of groundwater
–Groundwater is often mildly acidic
•Contains weak carbonic acid
•Dissolves calcite in limestone
–Caverns
•Formed by dissolving rock beneath Earth’s surface
•Formed in the zone of saturation
•Features found within caverns
–Form in the zone of aeration
–Composed of dripstone
–Calcite deposited as dripping water evaporates
–Common features include stalactites (hanging from the ceiling) and stalagmites
(growing upward from the floor)
–Karst topography
•Formed by dissolving rock at, or near, Earth’s surface
•Common features
–Sinkholes – surface depressions
–Sinkholes form by dissolving bedrock and cavern collapse
–Caves and caverns
•Area lacks good surface drainage