1

Chapter 10

Streams and Floods

GEOL 101

Introductory Geology

2

Hydrologic cycle

The hydrologic cycle is a summary of the circulation of Earth’s water supply

Processes involved in the hydrologic cycle• Precipitation• Evaporation• Infiltration• Runoff• Transpiration

3

The Hydrologic Cycle

4

Channel flow and sheet flowLongitudinal profile of a stream

Head area erosion predominant

Mouth area, deposition mainly

5

Downcutting

6

Drainage Pattern

Dendritic Radial

TrellisRectangular

7

Dendritic Drainage

8

Factors affecting erosion and deposition

VelocityGradient – rise over runChannel shapeChannel roughnessDischarge – amount of water flow per unit time, as discharge increases, load increases.

9

Velocity

10

Velocity

11

Channel shape and roughness

SemicircularFlow fast

Wide channelFlow slow

Rough bottomFlow slow

12

Channel shape and roughness

Narrow channelFlow fast

Wide channelFlow slow

13

Stream ValleysMost common landform on Earth’s surfaceTwo general types of stream valleys

• Narrow valleys– V-shaped – Downcutting toward base level– Features often include rapids and waterfalls

• Wide valleys– Stream is near base level

– Downward erosion is less dominant

– Stream energy is directed from side to side forming a floodplain

14

Stream Valleys

Features of wide valleys often include:• Floodplains

– Erosional: river erodes laterally

– Depositional: fluctuation in conditions, base level

• Meanders: sweeping bends in river channel– Cut bank: active zone of erosion

– Point bar: zone of deposition

– Cutoffs: shortened channel segment

– Oxbow lakes: abandoned bend

15

Stream Valleys

Features of narrow valleys often include: Rapids & Waterfalls

Both occur where stream profile drops rapidly,

• Rapids: resistive bed acts as temporary base level upstream, downcutting downstream

• Waterfalls: stream makes vertical drop– Resistive rock underlain by erosive rock

– Water plunges and erodes the underlying rock

– Niagra Falls

16

Changes from Upstream to Downstream

• Profile– Cross-sectional view of a stream– Viewed from the head (headwaters or source) to the

mouth of a stream– Profile is a smooth curve– Gradient decreases downstream

• Factors that increase downstream– Velocity– Discharge– Channel size

• Factors that decrease downstream– Gradient– Channel roughness

17

Stream erosion

Lifting loosely consolidated particles• Abrasion: particles scrape, rub, bump

together and wear down• Dissolution: dissolve soluble rock by

chemical reaction

Stronger currents lift particles more effectively

18

Active Stream Erosion

19

Base level and graded streams• Base level: lowest point to which a stream

can erode

• Two general types of base level– Ultimate (sea level)

– Local or temporary

• Changing conditions causes readjustment of stream activities

– Raising base level causes deposition

– Lowering base level causes erosion

20

Adjustment of Base Level to Changing Conditions

21

Local Base Level (waterfall)

22

Sediment Transport by Streams

RollingSuspensiondissolution

23

Sediment Deposition by Streams• Caused by a decrease in velocity

– Competence is reduced– Sediment begins to drop out

• Stream sediments– Generally well sorted– Stream sediments are known as alluvium

• Channel deposits– Bars– Braided streams– Deltas

24

Sediment Deposition by Streams• Floodplain deposits

– Natural levees: form parallel to the stream channel by successive floods over many years

– Back swamps: marsh

• Alluvial fans– Develop where a high-gradient stream leaves a

narrow valley

– Slopes outward in a broad arc

• Deltas– Forms when a stream inters an ocean or lake

– Consists of three types of beds (Foreset, Topset, Bottomset)

25

Deposition

Bar

26

Braided stream

27

Braided RiverResurrection River, AK

28

Erosion and Deposition Along a Meandering Stream

29

Meander Loop on the Colorado River

30

Flood plains

31

Stream Valley in Arid RegionOwens Valley, CA

32

Creation of oxbow lake

Creation of oxbow lake

33

Formation of a Delta

34

Deltasouthern, AK

35

Nile and Mississippi DeltasBird-FootTriangle shape

36

Alluvial FanDeath Valley, CA

37

FloodsFloods are the most common and most destructive geologic hazard

Causes of flooding• Naturally occurring• Human-induced factors

Types of floods• Regional floods• Flash floods• Ice-jam floods• Dam failure

38

FloodingSalt River, AZ

39

FloodsWhat is a 100-year flood?

It is not a flood that occurs every 100 years

Flood of a given size that has the probability of 1 in 100 of occurring in that year

Better term: 1-in-100 chance flood

Urban planning based on FEMA 100-yr flood maps

40

Flood ControlEngineering efforts

• Artificial levees• Flood-control Dams• Channelization

Nonstructural approach through sound floodplain management

• Identify high risk areas• Zoning regulations for development

41

Flood-Control DamsStore water for slow releaseLowers crest of flood, spread out over time Often have other functions• agricultural irrigation water• hydroelectric power• recreation

Reservoir covers previous land use: fertile farmland, historic sites, scenic valleysSediment deposition behind damImpediment for fish migration

42

ChannelizationAltering a stream channel to speed flow of water to prevent reaching flood height• Clearing channel of debris• Dredging to widen and deepen channel• Lining channel with concrete

Artificial cutoff: straightening the channel• shorter stream increases gradient and velocity

of water flow• larger discharge associated with flooding

dispersed more quickly

Army Corp of Engineers

43

Artificial LeveesEarthen mounds on river banks to increase the volume capacity of river

Steeper slopes than natural levees

Trap sediment that otherwise would have been deposited in floodplain

River bed build up often requires raising the height of levee over time

Many artificial levees not built to withstand extreme flooding

Levee failure numberous on Miss. (1993)

44

Erosional Floodplain

45

Incised Meanders and Stream Terraces

• Incised meanders– Meanders in steep, narrow valleys– Caused by a drop in base level or uplift of the

region

• Stream Terraces– Remnants of a former floodplain– River has adjusted to a relative drop in base

level by downcutting– Results in horizontal plane above current

floodplain

46

Stream terraces

47

Incised MeandersDelores River, CO

48

Incised MeandersColorado River, Canyonlands NP, UT