Chapter 14 Streams and Floods

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Terms in this set (...)

stream
a ribbon of water that flows in a channel
channel
a trough dug into the ground surface by flowing water
headwaters
the beginning point of a stream
near this point:
-gradient is steep
-discharge is low
-competence is high (sediments are coarse)
-channels are straight
mouth
the outlet of a stream where it discharges into another stream, lake or sea
flood
an event during which the volume in a stream becomes so great that it covers areas outside the streams normal channel
running water
surface water that flows down sloping land in response to the pull of gravity
streams banks
edges of a channel
streambed
floor of a channel
reach
defined length of a channel
upstream
closer to the source or headwaters of the stream
downstream
closer to the end or mouth
where the water in a stream comes from
Water enters the hydrologic cycle by evaporating from the Earth's surface and rising into the atmosphere. After a relatively short residence time, atmospheric water condenses and falls back to the Earth's surface as rain or snow that accumulates in various reservoirs.
sheetwash
film of water less than a few mm thick that covers the ground surface during heavy rains
groundwater
sinks in the ground where it becomes trapped in the soil or descends below the water table
water table
the level below which groundwater fills all the pores and cracks in subsurface rock or sediment
inputs
inputs to a stream comes from underground
substrate
material it flows over
downcutting
process in which water flowing through a channel cuts into the substrate and deepens the channel relative to its surroundings
runoff
all water flowing on the surface
-includes sheetwash plus the water in streams
headward erosion
The process by which a stream channel lengthens up its slope as the flow of water increases
tributaries
-side channels
smaller stream that flows into a bigger stream
drainage network (basin)
array of interconnecting streams that together drain an area
dendritic
When rivers flow over a fairly uniform substrate, they develop a dendritic network that looks like the pattern of branches connecting to the trunk of a deciduous tree
radial
drainage networks forming on the surface of a cone-shaped mountain flow outward from the mountain peak
rectangular
places where a rectangular rid of fractures break up the ground, channels follow pre-existing fractures and streams join each other at right angles
trellis
major tributaries flow down a valley and join a trunk stream that cuts across the ridges
parallel
on a steep uniform slope, several streams with parallel course develop simultaneously
watershed
region that collects water that feeds into a given drainage network
drainage divide
highland or ridge that separates one watershed from another
permanent streams
stream that flows year round because its bed lies below the water table
-can exist because:
they receive a supply of water from upstream and surface run off
:fill with groundwater seeping through the stream bed or channel walls
(Mississippi river, Amazon river)
ephemeral (intermittent) stream
stream who's bed lies above the water table, so that the stream flows only at the rate at which water enters the stream from rainfall or meltwater exceeds the rate at which water infiltrates the ground below
discharge
volume of water in a conduit or channel passing a point in 1 second
Formula : D = Ac× va
Ac:area
va:average velocity
souring
running water can remove loose fragments of sediment
breaking and lifting
-push of flowing water can break chunks of solid rock off the channel floors or walls
-flow of a current over a clast can cause the clast to rise
abrasion
sediment-laden water acts like sandpaper and grinds or rasps away at the channel floors and walls

--pothole: In places where turbulence produces long-lived whirlpools, abrasion by sand or gravel carves a bowl-shaped depression and forms a pothole
dissolution
running water dissolves soluble minerals, and carries the minerals away in solution
dissolved load
ions dissolved in a streams water
suspended load
tiny solids grains carried along by a stream without settling to the floor of the channel
bed load
large particles, such as sand, pebbles, or cobbles, that bounce or roll along a stream bed
saltation
movement of sediment in which grains bounce along their substrate, knocking other grains into the water column in the process
competence
maximum partial size a stream can carry
capacity
total quantity of sediment a stream can carry
alluvium
sorted sediment deposited by a stream
bar
-sheet or elongate lens or mound of alluvium
-unit of air-pressure measurement approximately equal to 1 atm
point bars
wedge shaped deposit of sediment on the inside bank of a meander
floodplain
flat plain on either side of a stream that becomes covered with water during a flood
delta
wedge of sediment formed at a river mouth when the running water of a stream enters standing water, the velocity slows, the stream loses competence, and the stream divides into a fan of distributaries
stream gradient
slope of a streams channel in a downstream direction
rapids
turbulent water plunges over a steep, bouldery bed
longitudinal profile
cross-sectional image showing the variation in elevation along the length of a river
base level
lowest elevation a stream channel's floor can reach at a given locality
near streams base level:
-gradient is nearly flat
-discharge is high
-competence is low (sediments are fine)
-channels curve and twist
local base level
location upstream of a steams mouth
ultimate base level
lowest possible elevation along the streams longitudinal
canyon
trough or valley with steeply sloping walls, cut into the land by a stream
valley
trough with sloping walls, cut into the land by a stream
stream terrace
When a stream downcuts through the alluvium of a floodplain so that a new, lower floodplain develops and the original floodplain becomes a step-like platform
waterfall
place where water drops over an escapement
braided stream
sediment choked stream consisting of entwined sub channels
-sediment load is very high and channel is very shallow

-abundant course sediment moves during floods
-sediment load is very high and channel is very shallow
-weaving channels create ephemeral sand and gravel bars
alluvial fan
gently sloping apron of sediment dropped by an ephemeral stream at the base of a mountain in arid or semiarid regions
meander
snake-like curve along a streams course
meandering stream
reach of a stream consisting of many meanders
-stream gradient is low
-substrate is soft and easily eroded
-the stream exists within a broad floodplain
thalweg
fastest moving part of a current swings back and forth
owbow lake
meander that has been cut off yet remains filled with water
natural levees
pair of low ridges that appear on either side of a stream and develop as a result of the accumulation of sediment deposited naturally during flooding
stream rejuvenation
renwed downcutting of a stream into a floodplain or peneplain, caused by a relative drop of the base level
steam piracy
process that happens when headward erosion by one stream causes the stream to intersect the course of another stream and capture its flow
drainage reversal
when the overall direction of a flow in a drainage network becomes the opposite of once had been
seasonal foods
Floods that appear almost every year during seasons when rainfall is heavy or when winter snows start to melt
-"wet season"
flash flood
flood that occurs during unusually intense rainfall or as the result of a dam collapse, during which the floodwaters rise very fast
floodways
mapped region likely to be flooded, in which people avoid constructing buildings
recurrence interval
average number of years between floods of a particular size
-100 yr flood means 1% risk of such a flood in 1 year
pollution
At the mouths of major rivers, pollutants spill into the sea, carrying nutrients (particularly nitrogen and phosphorus) that feed microorganisms
dam construction
alters ecosystem of a drainage network by forming barriers
overuse of water
use of water has grown especially in response to the green revolution of the 60's
effects of urbanization on streams
changes both the volume of the water reaching the stream and the duration of lag time
effects of agriculture
river's sediment load increases significantly when farms replace forests nearby.
hydrologic cycle
major part:
-evaporation
-transpiration
-precipitation
-sublimation
-infiltration
-melting
-runoff
lobes
indicate long-lasting distributaries
-active lobes grown in size and elevation
-abandoned delta lobes no longer receive sediment
-floodwaters devastate property and ruin buildings
-floods occur when flow exceeds channel capacity
-water overflows the channel onto adjacent land
Case history: Mississippi and Missouri Rivers 1993
-excess rainfall and snowmelt across region
-summer floodwaters invaded huge areas
-covered 40,000mi2
-flooding lasted 79 days
-50 people died
-55,000 homes destroyed
-12 billion dollars in damage
Case history : Bif Thompsons Canyon, Colorado 1976
-torrential rains (7.5 inches in an hour)
-high, fast water scoured rocks and soil
-houses, bridges, and roads vanished : a 257 ton rock moved
-144 people died
Evaluating Flood Hazard
-Government agencies (FEMA)
-collect hydrologic data and make food-hazard maps
-1% annual probability (100 year floods)
-0.2% annual probability (500 year floods)
-maintain integrity of leaves and flood controlled structures