Chapter 10 Streams and Floods GEOS 101 4/23/2017 GEOL 101 Introductory Geology Chapter 10 Streams and Floods

GEOS 101 4/23/2017 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

The Hydrologic Cycle

Channel flow and sheet flow Longitudinal profile of a stream Head area erosion predominant Mouth area, deposition mainly

Downcutting

Drainage Pattern Radial Dendritic Trellis Rectangular

Dendritic Drainage

Factors affecting erosion and deposition Velocity Gradient – rise over run Channel shape Channel roughness Discharge – amount of water flow per unit time, as discharge increases, load increases.

Velocity

Velocity

Channel shape and roughness Semicircular Flow fast Wide channel Flow slow Rough bottom Flow slow

Channel shape and roughness Narrow channel Flow fast Wide channel Flow slow

Stream Valleys Most common landform on Earth’s surface Two 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

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

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

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

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

Active Stream Erosion

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

Adjustment of Base Level to Changing Conditions

Local Base Level (waterfall)

Sediment Transport by Streams Rolling Suspension dissolution

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

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)

Deposition Bar

Braided stream

Braided River Resurrection River, AK

Erosion and Deposition Along a Meandering Stream

Meander Loop on the Colorado River

Flood plains

Stream Valley in Arid Region Owens Valley, CA

Creation of oxbow lake Creation of oxbow lake

Formation of a Delta

Delta southern, AK

Nile and Mississippi Deltas Triangle shape Bird-Foot

Alluvial Fan Death Valley, CA

Floods Floods 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

Flooding Salt River, AZ

Floods What 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

Flood Control Engineering efforts Artificial levees Flood-control Dams Channelization Nonstructural approach through sound floodplain management Identify high risk areas Zoning regulations for development

Flood-Control Dams Store water for slow release Lowers 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 valleys Sediment deposition behind dam Impediment for fish migration

Channelization Altering 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

Artificial Levees Earthen 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)

Erosional Floodplain

Incised Meanders and Stream Terraces 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

Stream terraces

Incised Meanders Delores River, CO

Incised Meanders Colorado River, Canyonlands NP, UT