Watershed and its divide

What’s the situation at points A and B in terms of: a. gradient b. stream velocity c. erosive power?

Headwaters

What does a stream carry and how?

• Total load (visible and invisible)– Suspended load (~ 90% of total, by weight)– Bed load (~ 10% of total)– Dissolved load (chemical ions)

• Discharge– Volume of water passing a point in a given

amount of time; usually cubic feet (or meters) per second

– Depends on cross sectional area; becomes higher when area is reduced

Note the asymmetry of the channel and the stream bed. Where is the fastest velocity and why?

What size particles are carried in the different locations in the water?

2

31

4

Characteristics of a meandering river

Anatomy of a stream

• Where do velocity lines lie in the stream?

• Where do erosion and deposition take place?

Depositional feature – alluvial fan

Depositional feature - delta

Changes in sinuosity (curvedness)

Entrenched stream

Meandering stream

What differences do you notice in these two rivers?

D Best

Braided streams

High sediment load

Floodplains

Floodplains

Erosion and deposition along meanders

floodplain

Formation of a cutoff

Note how increased sinuosity causes an eventual cutoff due to the stream taking the shortest course

Natural levees

Levees- natural and artificial

Types of drainage patterns

Types of floods• Flash

– Localized areas, usually thunderstorms; common in deserts

– Examples:• Antelope Canyon, AZ August 1997 11 died• Big Thompson Canyon, CO July 1976 145 died• Need to consider where storms are—seldom directly

overhead but up drainage area

• Regional, downstream– Prolonged rain which saturates ground or delayed

snowmelt– Floodplain covered

Flash flood vs downstream flood

Typically a narrowChannel; water risesFast; usually flash type

Wider area, oftenfarmland that getsflooded; generallyregional type

Flood frequency

• Recurrence interval– Length of time in years separating floods of a similar

size [measured as volume of flow of stream]– Larger floods have a longer time interval – We speak of the 100-yr flood, one that has a 1%

probability of occurring once in a given year– 20-yr flood would have a 5% likelihood of occurring in

a given year– Oak Creek to the south of Flagstaff experienced three

100-yr floods in a decade due to combinations of rare conditions

Flood activity in the U. S.

Large rivers in conterminous U.S.

Mississippi River basin facts• Mississippi River drains 42% of the U. S.• Greatest inundation floods in US occur here• 3rd largest river basin in world• 11 of 28 largest rivers in US are part of the

Mississippi River system• Avg flow 645,000 cu ft/sec• 1 cu ft = ~7.5 gal, so this is 4.83 million gals/sec

or 100+ swimming pools PER SEC

Drainage of Mississippi River

Mississippi River flooding 1992-93

Effects of flooding

Major flooding between 1993 and 1997

Missouri River in 1993

Our response to flooding• Dams

– Only provide some flood protection—not control; if full, lakes must be drained; failure of dam is major problem

• Levees– Create false sense of security; if water breeches levee, it doesn’t

drain well• Sandbagging

– Very short term solution• Zoning and insurance

– Can’t build in 100-yr floodplain; most people don’t buy floodplain insurance knowing fed govt will bail them out

• Channelization– Clear, deeper, wider, straighter paths for water – everything goes

against idea of graded stream [one seeking equilibrium]