Welcome to GLG101C Introduction to Geology Fall 2004 Professor James Tyburczy
Department of Geological Sciences
	Chapter 14 Streams

Chapter 14 Streams: Transport to the Oceans

Check out the Water Resources web site of the US Geological Survey

Stream: a body of running water confined to a channel moving downhill under the force of gravity

• Streams are the most important erosional force, the dominant shaper of landscape, and have a great impact on people through discharge and flooding
• Precipitation - Runs off (into streams), infiltrates into the ground, evaporates, or transpires through plants. On average 15 - 20 % of precipitation ends up in streams (run off)
• Stream runoff is influenced by the intensity and duration of rainfall, gradient, permeability (texture) of soil, vegetation, prior wetted condition of soil, temperature & humidity
• Streamflow - is generally turbulent (chaotic, very irregular) as opposed to laminar (smooth, predictable). This idea is important because turbulent flow can move much larger volumes of sediment than laminar flow.

Transport of sediment by streams

• Bed load - large materials transported along the bottom of the stream
  • Saltation - materials are picked up, moved downstream a small distance, return to the bottom, picked up again….(sand, gravel)
  • Traction - sliding, dragging along the bottom (boulders, cobbles)
• Suspended load - often the largest portion of the load - small (clay, silt-sized) solid particles are kept from settling to the bottom by turbulent flow
• Dissolved load - dissolved salts resulting from chemical weathering
• Different streams carry different proportions of the different types of sediment, depending on nature of materials over which they flow, the velocity, the discharge, and the gradient
• Carrying capacity - total amount of sediment that a stream can carry - depends on velocity, discharge, gradient
• Competence of a stream - a measure of the size of particles that a stream can move

Stream Erosion

• Abrasion - grinding, the most important
• Physical and chemical weathering
• Undercutting - erodes out softer layers (Niagra Falls)

Stream Valleys and flood plains

• Formation of flood plains and natural levees by recurring flooding and channel migration
• Deposition of sediment by streams occurs when stream loses velocity, often leads to sorting of sediments
• Alluvium - stream deposited sediment

Channel deposits - Sand and gravel bars, cross beds. Ripples and ripple marks occur in slow and moderate streams, larger dunes occur in higher velocity streams

Channel Patterns

• Straight channels
• Braided streams - areas of highly variable flow, heavy sediment load, wide shallow channel, easily eroded banks - for example here in the Basin and Range part of Arizona, or at the melting ends of glaciers
• Meandering streams - found in areas of more uniform stream flow, lower sediment load, lower gradient
• Velocity profile of meandering stream - faster near outer bank, slower near inner bank, leads to formation of point bars, cut banks, and oxbow lakes

Discharge - volume of water per unit time past a given point

• Width x Depth x Velocity

Flood frequency curves - average recurrence interval for a flood of a given size (estimated)

• Data only go back a few years (less than 100 in many cases)
• Man is constantly changing river channels (channelizing the river, building levees), and so changing the probabilities of big floods. Often we change the channels without knowing the long term effects or the effects in distant areas.
• Effects of urbanization on discharge and flooding
  • Increased runoff leads to more flooding
  • Decreased infiltration
  • Decreased time lag between time precipitation and time of peak flow or flooding

Longitudinal Profiles - elevation of stream versus distance from the headwaters down to the sea

• Dynamic Equilibrium between erosion and deposition
• Base level - lowest elevation to which a stream can erode its channel
• Presence of a lake changes the local base level for the stream, changes erosion and deposition patterns
• Factors influencing stream velocity and therefore erosional capacity
  • Gradient (slope) - typical stream profile is steeper at headwaters, gentler slope at mouth
  • Amount of discharge - velocity greater with greater discharge

   

Drainage patterns and deposits

• Alluvial fans - deposited where narrow streams leave mountains and go into wide valleys
  • Coarse grained, angular, poorly sorted
• Deposition occurs because the stream spreads out (widens its channel), slows down, and deposits coarse sediment
• Stream Terraces - downcutting caused by
  • Tectonic uplift
  • Climate change - increased river flow
  • Change of base level (lowering of sea level (for example, due to ice ages) - long term climate change)
• Drainage patterns -
  • Dendritic if material is relatively uniform
  • Can be radial, rectangular, or trellis in form if controlled by geologic structures and/or resistant layers
• Deltas - distributaries carry water from main channel to the sea
  • Characteristic sediments in different parts of the delta - coarser (sand) nearer the shore, silt farther out
• Deltas change rapidly over time, even human time scales - man has built infrastructure that depends on river channels and deltas staying where they are now. Natural tendency of the river is to constantly change the pattern. It is a constant battle to maintain rivers and streams where we want them (as opposed to where they want to go).