GEOL 1001 Chapter : Chpts 14 18
Geology 1001
Final Exam Notes
Chapter Fourteen – Streams and Floods
• Introduction – Water that falls on land drains back to the sea via rivers. Earth is
the only planet in the Solar System that currently has streams of running water.
Streams are ribbons of running water across the land and flow down channels.
Channels are troughs cut into the land that confine streams and running water.
Rivers are large streams. Streams remove (drain) excess water (runoff) from the
landscape and carry it eventually to the sea. During this process, streams: Erode
the landscape, transport sediment and nutrients, leave behind deposits of
sediment, provide habitat, provide avenues for commerce, water for agriculture
and sources of power. A flood is when the volume of water flowing within a
stream channel exceeds the volume of that channel; water then covers an area
that would normally be dry land. Stream runoff is a component of the hydrologic
cycle. Hydrologic cycle processes: evaporation, transpiration, precipitation,
infiltration and runoff.
• Forming Streams – Stream flow begins as water is added to the surface, either
by melted snow, snow or rainwater. A stream flow begins as moving sheetwash
(thin surface layer of water). This sheetwash then moves down the steepest
slope, where gravity pulls water downhill. It erodes substrate as the pressure of
the weight of the water squeezes water from the soil and carries sediment with
it (eroding the channel). Sheetwash erosion initially creates tiny rill channels.
Rills coalesce, deepen and downcut into channels. Intense scouring marks entry
into the channel. Rapid erosion lengthens the channel upslope = headward
erosion. Over time, nearby channels merge. Smaller tributaries (side channels)
join a larger trunk stream. The array of linked channels is a drainage network.
These networks reach to all corners of a region to provide conduits for the
removal of water. Drainage networks change over time depending on the shape
of the landscape and composition of the substrate.
• Drainage Networks – The array of linked channels is a drainage network.
Drainage networks often form geometric patters. These patterns reflect
underlying geology. Common drainage patterns are: dendritic, radial,
rectangular, trellis or parallel. Dendritic is a branching, “treelike” pattern due to
uniform material and slope. Radial patterns form from a cone-shaped mountain
or point of uplift. Jointed rocks control rectangular patterns, where channels
form along the preexisting fractures. Trellis patterns form by alternating
resistant and weak rocks, usually within landscapes of parallel valleys and ridges.
Parallel patterns form when streams develop on a uniform slope, where several
streams run parallel to one another.
• Drainage Basins – Drainage basins are land areas that drain into a specific trunk
stream. It is also known as catchments or watersheds. Drainage divides are
uplands (such as a highland or ridge) that separate drainage basins. Watersheds
exist in a variety of scales: tiny tributaries or continental rivers. Large watersheds
feed large rivers and section continents. Continental divides separate flow to
different oceans.
• Stream Longevity – Once a river or stream forms, does this remain a permanent
feature of Earth’s surface? Yes and no. Permanent streams flow all year long,
whilst ephemeral streams flow only for parts of the year (or only for a brief
period of time following heavy rains). Permanent streams: water flows all year,
at or below water table, in humid or temperate climates, sufficient rainfall, lower
evaporation, seasonal discharge variation. Ephemeral streams: do not flow all
year long, above water table, in dry climates, low rainfall, high evaporation, flow
mostly during rare flash floods.
• Flows in Streams – Discharge is the amount of water flowing in a channel. The
volume of water passing a point per unit. It varies seasonally due to precipitation
and runoff. Velocity is not uniform in all areas of a channel. Friction slows water
along channel edges. Friction is grater in wider, shallower streams. Lesser in
narrower, deeper streams. In straight channels, highest velocity is in the center.
Few neutral channels are straight. In curved channels, maximum velocity traces
the outside curve. The outside curve is preferentially scoured and deepened. The
deepest part of the channel is called the thalweg. Flow around curved channels
follows a spiral path. Stream flow is characteristically turbulent. Chaotic and
erratic. Abundant mixing. Swirling eddies. High velocities. Turbulence is cause by
flow obstructions and shear in water.
• Erosional Processes – The energy imparted to stream flow is derived from
gravity. Streams do work by converting potential to kinetic energy. Erosion is
maximized during floods. Large water volumes. High water velocities. Abundant
sediment. Running water causes erosion in four ways: scouring, breaking and
lifting, abrasion and dissolution. Scouring is when running water picks up
sediment and moves it. Braking and lifting is the force of moving water, which
can break chunks of rock off the channel and lift rocks off the channel bottom.
Abrasion is when sediment grains in flow sandblast rocks. Bedrock exposed in
channels is often polished smooth. Gravel swirled by turbulent depressions is
called potholes. Potholes are often intricately sculpted. Dissolution occurs when
mineral matter dissolves in water. The degree of erosion depends on the velocity
and volume of water and the sediment content. Large volume of fast-turbulent
sandy water will cause more erosion than slow-moving quite, clear water. Most
erosion occurs during floods – supplying streams with large volumes of fast-
moving, sediment-laden water.
• Sediment Transport – Sediment load is the material moved by streams. There
are three types of load: dissolved load – ions from mineral weathering,
suspended load – fine particles (silt and clay) in the flow. Bed load, which is
when larger particles roll, slide and bounce along. Sediment transport changes
with discharge. High discharge involves large cobbles and boulders moving. Low
discharge involves large clasts, which are stranded. Competence is the maximum
size transported. Capacity is the maximum load transported.
• Sediment Deposition – When velocity decreases so does competence. Sediment
grains drop out; water sorts them by size. Gravel settles in channels. Sands are
removed from the gravels; mud from both. Sands drop out in near channel
environments. Silts and clays are suspended only to settle in slack water. Fluvial
sediments are called alluvium. Channels may be decorated with mid-channel
bars. Sands build up the point bars inside meander bends. Velocity of flow
affects the size of clasts carried. Large clasts tend to be carried during floods.
Smaller clasts (such as sand and mud) are characteristics of typical flow
velocities. Muds are deposited away from the channel during floods – often
along flood plains. A stream builds a sediment delta upon entering a lake or large
body of water.
• Longitudinal Changes – The character of a stream changes with flow distance. In
profile, the gradient describes a concave-up curve. The character of a stream
changes with flow distance. Near the headwater source of the stream gradient is
steep, discharge is low, streams are coarse and channels are straight and rocky.
The character of a stream changes with flow distance. Toward the mouth
gradient flattens, discharge increases, grain-sizes are smaller and channels
describe broad meander belts. From the headwaters to the mouth, a river
changes in slope, shape and flow characteristics.
• Base Level – A base level is the lowest point to which a stream can erode.
Ultimate base level is defined by the position of sea level. Streams cannot erode
below sea level. A lake serves as a local (or temporary) base level. Base level
changes cause stream re-adjustments. Raising base level results in an increase in
deposition. Lowering base level accelerates erosion. A stream’s mouth cannot be
lower than the base level. Base level controls: a ledge of resistant rock, which
may define the local base level. This results in waterfalls! ☺ Erosive forces act to
slowly remove the resistant layer. This acts to restore the longitudinal profile.
• Streams and Landscape – Streams sculpt Valleys and canyons; land far above
base level is subject to down cutting, creating an eroded trough. A canyon is
steep trough sidewalls forming cliffs and valleys are gently sloping trough
sidewalls with a defined V-shape. Valleys or canyons are determined by the rate
of erosion versus the strength of the rock. If down cutting by the stream happens
faster than mass wasting on the walls a slot canyon forms. The canyon widens as
the stream undercuts the walls. If mass wasting takes place as fast as down
cutting occurs, a V-shaped valley develops. Stratigraphic variation often yields a
stair step profile. Strong rocks yield vertical cliffs. Weak rocks produce sloped
walls. Geologic processes stack strong and weak rocks. Active down cutting
flushes sediment out of channels. Valleys store sediment when base level is
raised. The surface of this alluvium-deposit becomes a broad floodplain.
Renewed incisions from the base level dropping or increased discharge, stream
terraces are created. Terraces mark former floodplains. Rapids consist of
