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Chapter 17
RiversRivers
Chapter 17
Outline• Streamflow
-streams/rivers, runoff, hydrologic cycle component-forming streams/rivers
• Drainage networks-Patterns (dendritic to trellis)-Drainage basins, drainage divides
• Rivers/Streams-Permanent vs. ephemeral-Discharge, channel velocity, erosion/transport/deposition-Longitudinal (downstream) changes
• Further details-Base level, valleys & canyons, terraces, rapids/waterfalls-Depositional environments (e.g. alluvial fans, braiding, deltas..)-Drainage evolution and flooding
Chapter 17
Chapter 17
Streamflow• Stream/River – • Runoff – • Stream runoff is crucial for humans:
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Streamflow• Stream runoff also
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Streamflow• Stream runoff is an important
• Flowing water…
• Earth: only planet
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The Hydrologic Cycle• Stream flow –
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Forming Streams• Streamflow begins
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Forming StreamsStreamflow begins as
• Sheetwash erosion
• Rills coalesce & deepen
into channels.
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Forming Streams
• Scouring can mark entry into the channel • Rapid erosion lengthens channel upslope
• Process is called headward erosion
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Forming Streams• Over time, channels merge. • Smaller tributaries join larger trunk steam• A drainage network –array of linked channels
Chapter 17
Outline• Streamflow
-streams/rivers, runoff, hydrologic cycle component-forming streams/rivers
• Drainage networks-Patterns (dendritic to trellis)-Drainage basins, drainage divides
• Rivers/Streams-Permanent vs. ephemeral-Discharge, channel velocity, erosion/transport/deposition-Longitudinal (downstream) changes
• Further details-Base level, valleys & canyons, terraces, rapids/waterfalls-Depositional environments (e.g. alluvial fans, braiding, deltas..)-Drainage evolution and flooding
Chapter 17
Chapter 17
Drainage Networks• Drainage networks form geometric patterns• Patterns reflect geolgoy and landscape form• Several common drainage patterns
1. Dendritic – branching, tree like, due to uniform material
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Drainage Networks
• Common drainage patterns:2. Radial – form a point uplift
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Drainage Networks
• Common drainage patterns:3. Rectangular – controlled by jointed rocks
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Drainage Networks• Common drainage patterns:
4. Trellis (garden) – due to atlernating resistant/weak rocks
common in fold-in thrust belts
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A Drainage Basin
• Land area that drains into a specific trunk stream Also called catchment or watershed
• Divides are boundaries that separate drainage basins
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Drainage Divides
• Watersheds exist across scales.
• Tiny tributaries • Continentals rivers• Large watersheds…• Feed large rivers• Section continents• Continental divides
separate flow to different oceans
Chapter 17
Outline• Streamflow
-streams/rivers, runoff, hydrologic cycle component-forming streams/rivers
• Drainage networks-Patterns (dendritic to trellis)-Drainage basins, drainage divides
• Rivers/Streams-Permanent vs. ephemeral-Discharge, channel velocity, erosion/transport/deposition-Longitudinal (downstream) changes
• Further details-Base level, valleys & canyons, terraces, rapids/waterfalls-Depositional environments (e.g. alluvial fans, braiding, deltas..)-Drainage evolution and flooding
Chapter 17
Chapter 17
Permanent vs. Ephemeral
• Permanent streams• Water flows all year.• At or below the water table.• Humid or temperate.
• Sufficient rainfall.• Lower evaporation.
• Discharge varies seasonally.
• Ephemeral streams• Do not flow all year.• Above the water table.• Dry climates.
• Low rainfall.• High evaporation.
• Flow mostly during rare flash floods.
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• Amount of water flowing in a channel • Water volume passing a point per unit time
• Cubic meters per second • Given by cross-sectional area• Varies seasonally due to precipation and runoff
Discharge
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Channel Velocity• Velocity is
• Friction• Greater in• Lesser in• Magnitude determined by
• In straight channels,
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• Velocity is not uniform within a channel• Max. velocity near outside in bending channels
• Outside is preferentially scoured and deepened• Inside is locus of deposition due to reduced velocity • Deepest part is called the thalweg
Channel Velocity
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• Velocity is not uniform in all areas of a channel • Stream flow is turbulent• Chaotic
• Turbulence caused by…• Flow obstructions• Shear in water• Eddies scour channel
bed.
Channel Velocity
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Erosion Processes• River flow does work
• Energy imparted is derived from gravity • Do work by converting potential to kinetic energy
• Erosion is maximized during floods
Large water volumes, high velocties
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Erosion Processes• Stream erosion:
1. Scouring –
2. Breaking & lifting –
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Erosion Processes3. Abrasion –
• Exposed bedrock in channels gets• Gravel swirled by turbulent eddies
• Bowl-shaped depressions are called
4. Dissolution –
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Sediment Transport
• Sediment load -• 3 types:
1. Dissolved load –
2. Suspended load –
3. Bed load –
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Sediment Transport
• Competence – • Capacity –
• Change with discharge:• High discharge – • Low discharge –
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Sediment Deposition• When flow velocity decreases…
• Competence is• Grain sizes are sorted by water.
• Sands are removed from gravels; muds from both. • Gravels settle in channels.• Sands drop out in near channel environments.• Silts & clays drape floodplains away from channels.
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Sediment Deposition
• Sediment size tracks with• Coarsest particles typify• Fine particles typify
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Sediment Deposition
Fluvial (river) sediments are called• Channels may have• Sands build up• A stream builds a
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Longitudinal Changes• Stream character• In profile, the gradient describes
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Longitudinal Changes• Near stream headwaters…
• Gradient is• Sediment sizes are• Channels are
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Longitudinal Changes
• Toward the mouth (downstream end)…• Gradient is• Smaller• Channels are
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Outline• Streamflow
-streams/rivers, runoff, hydrologic cycle component-forming streams/rivers
• Drainage networks-Patterns (dendritic to trellis)-Drainage basins, drainage divides
• Rivers/Streams-Permanent vs. ephemeral-Discharge, channel velocity, erosion/transport/deposition-Longitudinal (downstream) changes
• Further details-Base level, valleys & canyons, terraces, rapids/waterfalls-Depositional environments (e.g. alluvial fans, braiding, deltas..)-Drainage evolution and flooding
Chapter 17
Chapter 17
Base Level ConceptLowest point to which a stream
• Ultimate base level is
• A lake serves as a• Base level changes cause
• Raising base level results in an• Lowering base level
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Valleys and Canyons
• Land far above base level• Rapid down cutting creates
• Valley – • Canyon –
• Determined
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Stream Terraces
• Valleys store sediment when• Stability, then renewed
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Rapids & Waterfalls• Rapids are• Waterfalls are• Reflect geologic control:
• Flow over• Flow• Sudden increase
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Alluvial Fans
• Build at mountain front by• Sediments rapidly• Sediments create
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Braided Streams• Form where channels• Flow is forced around
• Diverging - converging flow• Bars are unstable
• Flow occupies
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Meandering Streams• Channels can form
• Along• Where streams travel over• When substrates are
• Meanders increase• Meanders
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Meandering Streams• Max velocity swings back &
• Fast water erodes• Point bar
• Meanders change due to natural variation in...• Thalweg• Get cutoff when
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Meandering Streams• Meanders become
• Cut bank erodes; point bar accretes.• Curves become
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Deltas
• Deltas form a river enters• Flow
• Channel divides into a
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Deltas
• Mississippi has a• Distinct lobes indicate• River periodically
• River breaks• Establishes a shorter, steeper path
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Drainage Evolution
• Streamflow is cause of most
• Example:• Uplift changes• Streams cut• Valleys• Landscape lowered
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Drainage Evolution
Stream piracy• One stream
• Results from• A stream with
• Captured
• Below capture point,
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Drainage Evolution
Drainage reversal• Tectonic uplift can• South America used to• Western uplift
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Drainage Evolution
Antecedent drainages• Tectonic uplift can raise
• If erosion keeps pace with uplift,
• Called
• If uplift rate exceeds incision,
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Drainage Evolution• Some antecedent streams
• Meanders initially develop• Uplift raises• Meanders incise
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Raging Waters• During a flood…
• Flow exceeds• Velocity (thus, competence & capacity)• Water leaves• Moving water & debris• Water slows away from the
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Raging Waters• Numerous causes of floods:
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Raging Waters
• Case history: Mississippi and Missouri Rivers, 1993.• Spring 1993:• July 1993:
• Covered 40,000 mi2.• Flood lasted 79 days.• 50 people died.• 55,000 homes destroyed.
• $12 billion in damage.
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Raging Waters• Seasonal floods recur on an annual basis.
• Monsoons – • Intense period of• Many people live in floodplain & delta plain settings.
• 1990 - monsoon killed 100,000 people in Bangladesh.
• 2008 – monsoon caused the Kosi river to avulse, displacing ~2.3 million people in Nepal/India.
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Raging Waters• Kosi River flood before and after.
• New channel width ~20 km!
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Raging Waters• Kosi River flood before and after.
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Raging Waters
• Ancient floods: Ice-Age megafloods.• 11 Ka, • Water scoured
• Created
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Living with Floods
• People living in floodplains
• Land use changes may• Establish floodways – • Remove
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Living with Floods
• Flood risk borne by
• Use hydrologic data to• Maps allow agencies to• Building in flood-prone settings is
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Living with Floods
• Flood risk is calculated• Discharges are plotted against• On semi-log, this plots as a• Probability (% chance of occurrence) given discharge will
happen
