Ch 18: River Streams

Stream (river)

Flowing water system transporting sediment from mountains to oceans 

Drainage basin

Area of land where all water drains into a single river system 

Mississippi River basin

Largest drainage basin in the United States 

Colorado River basin

Covers much of Arizona (example

Transboundary river

River forming borders between states or regions 

Red River

Forms Texas-Oklahoma border 

Drainage divide

Elevated boundary separating drainage basins 

Continental divide

Major divide separating large-scale drainage systems 

Drainage basin stability

Generally stable but can change due to erosion or climate 

Longitudinal profile

Graph of elevation vs distance along a stream 

Stream gradient

Slope decreases downstream causing slower flow 

Water flow direction

Always from high elevation to low elevation 

River valley

Entire area from ridge to ridge 

Channel

Portion of valley occupied by flowing water 

Floodplain

Flat area adjacent to channel where flooding occurs 

Floodplain sediments

Fine-grained silt and clay deposited during floods 

Sediment transport types

Suspended load, bed load, saltation 

Suspended load

Fine particles (clay, silt) carried within water column 

Bed load

Coarse particles (sand, gravel) rolling/sliding along bottom 

Saltation

Sediment transport by bouncing/jumping motion 

Transport vs velocity

Higher velocity allows transport of larger particles 

Capacity

Total amount of sediment a river can carry 

Competence

Maximum particle size a river can transport 

Velocity effect

Both capacity and competence increase with velocity 

Flow volume effect

Capacity increases with increasing discharge volume 

Heavy rainfall effect

Increases velocity, discharge, capacity, and competence 

Braided stream

Multiple interweaving channels separated by sediment bars 

Braided stream conditions

High energy, steep slopes, variable flow, high sediment load 

Poorly sorted sediment

Common in braided streams 

Meandering stream

Curved, sinuous river channel in low-energy environments 

Meander formation

Occurs in flat areas with low velocity 

Point bar

Sediment deposited on inside bend of meander 

Outer bank erosion

Faster current erodes outside of meander bends 

Inside bank deposition

Slower current deposits sediment forming point bars 

Meander migration

Side-to-side shifting of river bends over time 

Oxbow lake

Abandoned meander loop cut off from main channel 

Oxbow formation

Occurs when river cuts across a meander during flood 

Floodplain elevation

Can become higher than surrounding land over time 

Natural levee

Raised banks formed by sediment deposition during floods 

Floodplain sediment distribution

Coarse near channel, fine farther away 

Delta

Sediment deposit where river enters standing water (ocean/lake) 

Delta sediment

Sand and silt deposited near river mouth 

Clay transport

Finer particles carried farther into ocean forming shale 

Stream discharge (Q)

Volume of water flowing per unit time 

Discharge formula

Q = area × velocity 

Discharge units

cubic meters per second (m³/s) or cubic feet per second (CFS) 

Discharge increase downstream

Due to tributaries and groundwater input 

Cross-sectional area

Width × depth of stream channel 

High discharge factors

Larger area and higher velocity 

Flood

Occurs when discharge exceeds channel capacity 

Flooding cause

Excess precipitation and runoff input 

Flooding nature

Natural and expected in river systems 

Floodplain role

Stores excess water during floods 

Flood frequency

Likelihood of flood occurrence over time 

Recurrence interval

Average time between floods of a given magnitude 

100-year flood

Flood with 1% chance of occurring in any given year 

Flood probability

Determined using historical discharge data 

Example flood

1800 m³/s flow has ~10% yearly probability (~12-year interval) 

Mississippi River flood

Example of large-scale flooding event (1993) 

Hurricane Katrina (2005)

Major flood event causing levee failure 

Harvey (Houston)

Example of extreme rainfall increasing discharge 

Levee failure

Occurs when floodwaters exceed structural limits 

River energy

Controlled by slope and discharge 

High-energy streams

Steep slopes, fast flow, coarse sediment transport 

Low-energy streams

Flat slopes, slow flow, fine sediment deposition 

Tributary

Smaller stream feeding into a larger river 

Stream network

System of interconnected channels within drainage basin 

Sediment sorting

Process where particles are separated by size during transport 

Floodplain confinement

Artificial narrowing increases flood risk