chapt 16 geology

hydrologic cycle

unending circulation of earth’s water supply

evaporation infiltration

water moving into soil or rock and then evaporating through plants

runoff

water that flows over land rather than infiltrating it

river

streams with large amount of water

drainage basin

where rainfall and runoff collect to drain into river

headward erosion

extending the length of a river with converging surface flows

zone of sediment

where most of sediment is derived: headwaters region of river

sediment transport

acquired by a stream traveling through trunk streams collecting organic and inorganic particles

sediment deposition

when a river reaches the ocean, energy slows and sediment accumulates at the mouth of the river

drainage systems

interconnected networks of streams

dendritic pattern

treelike pattern forming where material beneath it is uniform

radial pattern

where streams diverge from a central area

rectangular pattern

many right angle bends developing when bedrock is crisscrossed by faults

trellis pattern

looks rectangular with tributary streams running parallel. Forms with bands of resistant/unresistant rock

water gap

steep walled notch followed by the river through a ridge of resistant rock

antecedent stream

streams cut across structures

superposed stream

eroded its channel into an existing structure

laminar flow

very slow stream with water moving in parallel and straight paths

turbulent flow

causes whirlpools and white water rapids

gradient

vertical drop of a stream over a specified distance

channel shape

measured by wetted perimeter (a slice taken across a channel)

channel size

a dynamic measurement a river adjusts to, to transport water/sediment

discharge

measure used to compare the size of streams. Volume of water flowing past a certain point at a given time

stage

height of water surface relative to a fixed reference point

longitudinal profile

cross sectional view of a stream from its source area (head) to its mouth (where it empties into a large body of water)

quarrying

removal of blocks from the bed of a stream channel by fracturing

abrasion

the bed and banks of a bedrock channel are bombarded by particles carried by the flow causing erosion

potholes

circular depressions common in bedrock channels

corrosion

rock is gradually dissolved by the flowing water, common in limestone bedrock

dissolved load

brought to stream by groundwater and is dispersed through out flow

suspended load

muddy/very fine sand. flooding can increase the amount in this load

settling velocity

the speed at which a particle falls through flow due to gravity

saltation

irregular skipping or jumping of rocks along stream bed

capacity

max load of solid particles a stream can transport per unit time

competence

a measure of a stream’s ability to transport particles based on size. a swfit stream has more competence than a slow one

alluvium

sediment deposited by streams

bedrock channels

cut into underlying strata and typically form in the headwaters of a river

alluvial channel

patterns reflect a stream’s ability to transport its load at a uniform rate while expanding the least amt of energy

meandering channels

transport much of their load in suspension generally moving in sweeping bands in deep channels

cut bank

area of active erosion outside a meander

point bars

crescent shaped accumulation of sand/gravel deposited on the inside of a meander

cutoff

new, shorter channel segment formed by river eroding into neck of land

braided channels

complex network of converging and diverging channels that thread their way among islands of gravel bars

stream valley

a channel/surrounding terrain that directs water into a stream

base level

downward limit to stream erosion

ultimate base level

sea level

local base level

the lowest point a river can erode its channel along a specific segment of its course

graded stream

a stream where its slope, channel discharge, and shape have changed over time to balance erosion and deposition

downcutting

occurs when stream’s gradient is steep and the channel is well above base level. Stream bed will deepen causing a V shape valley with steep sides

rapids

created by resistant beds acting as a temporary base level upstream while allowing downcutting to continue downstream

floodplain

caused by continuous lateral erosion by shifting meanders producing a broad/flat valley floor

incised meanders

some rivers have meandering channels that flow in steep narrow bedrock valleys

stream terraces

occurs after a river that has been flowing on a floodplain adjusts to a drop in base level, producing another floodplain

bars

small scale channel deposits composed of sand and gravel

delta

forms where sediment laden streams enters the still waters of a lake/inland sea/ocean

forset beds

coarse particles from the bed load deposited almost immediately upon entering the water to form layers that slope down towards delta

topset beds

thin horizontal, deposited during flood stage, covers forset bed

bottom set beds

finer silt/clays in suspended load settle away from the mouth

distributaries

carry water away from the main channel in varying paths to base level

mississippi delta

a series of seven subdeltas. formed when main flow was diverted from one channel to a short/direct path to gulf of mexico

leeves

built during european settlement. earthen mounds built parallel to the river. prevent sediment from being dispersed into wetlands

natural levees

built by years of successive floods. occurs when a stream overflows into floodplain as a broad sheet, dropping flow velocity with coarser portion of suspended load load being deposited next to channel

back swamp

area behind a levee that is poorly drained

yazoo tributaries

streams that flow through back swamps before finding an opening to rejoin the river

regional floods

seasonal with rapid melting or heavy rainfall being main factors

flash floods

rainfall intensity duration, surface conditions, and topography are main factors

ice jam floods

level of a stream rises, breaking up ice and creating ice floes that obstruct channels

dam failure floods

when larger floods occur the dam or levee may burst resulting in water behind it causing a flash flood

recurrence interval

an estimate of how often a flood of a given size can be expected to occur.

flood control dams

store flood water and let it out slowly

channelization

altering stream channel in order to make the flow more efficient (clearing obstructions/dredging)

non structural approach

many scientists today urge for floodplain management with zoning regulations