University:
California State University, Northridge
Course:
GEOL 101 | Geology of Planet Earth
Academic year:

2024
Views:

166

Pages:

26
Author:

hernomenvxp

Streams and Floods Physical Geology, Chapter 10 Running Water • Running water is the most important geologic agent in eroding, transporting and depositing sediment • Nearly every landscape on Earth shows the results of stream erosion or deposition Hydrologic Cycle • Hydrologic cycle - the movement and interchange of water between the sea, air, and land – Evaporation • Solar radiation provides energy – Precipitation • Rain or snow – Transpiration • Evaporation from plants – Runoff • Water flowing over land surface – Infiltration • Water soaking into the ground Running Water Stream - a body of running water, confined to a channel, that runs downhill under the influence of gravity – – • • Headwaters - upper part of stream near its source in the mountains Mouth - place where a stream enters sea, lake or larger stream Channel - a long, narrow depression eroded by a stream into rock or sediment Stream banks - sides of channel •Streambed – channel bottom Floodplain - flat valley floor composed of sediment deposited by the stream Before Flooding (August 14, 1991) After Flooding (November 7, 1993) Missouri and Mississippi River spill over into floodplain. green = vegetation, red = recently plowed fields (bare soil) Drainage Basins • Drainage basin - the total area drained by a stream and its tributaries – Tributary - a small stream flowing into a larger one • Divide - ridge or high ground that divides one drainage basin from another – Continental Divide separates the streams that flow into the Pacific from those that flow into the Atlantic and Gulf of Mexico Drainage Patterns • Drainage pattern - the arrangement, in map view, of a stream and its tributaries – Most tributaries join the main stream at an acute angle, forming a V or Y pointing downstream – Dendritic - drainage pattern resembling the branches of a tree – Radial pattern - streams diverge outward like the spokes of a wheel • Typically form on conical mountains (volcanoes) – Rectangular pattern - tributaries have frequent 90° bends and join other streams at right angles – Trellis pattern - parallel streams with short tributaries meeting at right angles Stream Erosion • Stream erosion (and deposition) controlled by flow velocity and discharge – Stream velocity controlled by stream gradient (slope), channel shape and channel roughness • Maximum velocity near center of channel – Floods involve increased velocity and discharge (volume of water passing a particular point in a stream over time) • Higher stream velocities promote erosion and transport of coarser sediments – Erosion of very small particles difficult due to molecular binding forces Stream Erosion • Stream gradient is the downhill slope of the streambed – Typically measured in feet per mile in the U.S., and in meters per kilometer elsewhere – Usually decreases downstream • Channel shape and roughness – – – – – Both effect stream velocity due to drag Narrower, deeper channels allow faster flow Smoother channels allow faster flow Wider, shallower channels decrease speed Rougher channels decrease flow speed • Stream discharge is the volume of water flowing past a given point in a unit of time Stream Erosion • Streams cut their own valleys, deepening and widening them over time and carrying away the sediment • Stream erosion occurs by three mechanisms: hydraulic action, solution, and abrasion – Hydraulic action - ability of flowing water to pick up and move rock and sediment – Solution - dissolving of rocks (e.g., limestone) – Abrasion - grinding away of stream channel by the friction and impact of the sediment load • Potholes are eroded into streambed by the abrasive action of the sediment load in the stream • Sediment load transported by a stream can be subdivided into bed load, suspended load, and dissolved load • Bed load - large or heavy particles that travel on the streambed – Traction load - large particles that travel along the streambed by rolling, sliding or dragging – Saltation load - medium particles (typically sand-sized) that travel downstream by bouncing along - sometimes in contact with the streambed and sometimes suspended in the flowing water • Suspended load - sediment that is small/light enough to remain above the stream bottom by turbulent flow for an indefinite period of time • Dissolved load - dissolved ions produced by chemical weathering of soluble minerals upstream Sediment Transportation Sediment Deposition • Sediments are temporarily deposited along stream course as bars and floodplain deposits, and at/near its end as deltas or alluvial fans • Bars - ridges of sediment (usually sand or gravel) deposited in the middle or along the sides of a stream – Braided streams contain sediment deposited as numerous bars around which water flows in highly interconnected rivulets (heavy sediment shallow, wide channel) Sediment Deposition • Meandering streams flow faster along the outside of bends and more slowly along the inside, depositing point bars on the insides of the meanders • Meander cutoffs may form when a new, shorter channel is cut through the narrow neck of a meander (as during a flood) Insert Fig. 10.20 Meander cut-offs “oxbow” Blackfoot River, Montana Sediment Deposition • Floodplains are broad strips of land built up by sedimentation on either side of a stream channel – Floodplain sediments are left behind as flood waters slow and recede at the end of flood events – Main channel has slightly raised banks with respect to the floodplain known as natural levees Sediment Deposition • Delta - body of sediment deposited at the mouth of a river when flow velocity decreases – Surface marked by shifting distributary channels – Shape of a delta depends on whether its wavedominated, tidedominated, or streamdominated Sediment Deposition • Alluvial fan - large, fan- or cone-shaped pile of sediment that forms where stream velocity decreases as it emerges from a narrow mountain canyon onto a flat plain – Well-developed in desert regions, such as the southwestern U.S. – Larger fans show grading from large sediments nearest the mountains to finer sediments farther away Flooding • When water levels rise and overtop the banks of a river, flooding occurs – Natural process on all rivers – Described by recurrence intervals • A 100-year flood is, on average, the size of the largest flood within a 100-year period of time – Can cause great damage in heavily populated areas – High velocity and large volume of water causes flood erosion – Slowing of waters as flood ends causes flood deposits (usually of silt or claysized particles) to be deposited in the floodplain Flooding • Urban flooding – Paved areas and storm sewers increase runoff by inhibiting infiltration – Rapid delivery of water to streams increases peak discharge and hastens occurrence of flood • Flash floods – Local, sudden floods of large volume and short duration – Typically triggered by heavy thunderstorms Flooding • Flood control – Dams designed to trap flood waters in reservoirs upstream and release it gradually over time – Artificial levees designed to increase capacity of river channel • Works well until stream overtops artificially raised levees, leading to extremely rapid flooding and erosion – Wise land-use planning, including prevention of building within 100-year floodplains, is most effective A Controlled Flood – Grand Canyon *Dams stop seasonal flooding. *Experiment to test bed scouring and deposition of sandbars/beaches: *Discharge from Glen Canyon Dam increased from 8000 to 45000 cfs for 6 days *Observation: Deep scouring of canyon, redeposition of beaches, boulders moved downstream Stream Valley Development • Downcutting – Process of deepening a valley by erosion of the streambed – V-shaped valleys typically form from downcutting combined with mass wasting and sheet erosion – Streams cannot erode below their base level • Basel level can be sea level, a lake, or the bottom of a closed basin (e.g., Death Valley, CA) • Downcutting rate can be rapid if a stream is well above base level (e.g., Grand Canyon, AZ) Stream Valley Development • Graded streams – Characteristic concave-up longitudinal profile – Rapids and waterfalls have been smoothed out by extensive erosion over a long period of time – Delicate balance between available sediment load and transport capacity • Lateral erosion widens stream valleys by undercutting of stream banks and valley walls as stream swings from side to side across the valley floor • Headward erosion is the slow uphill growth of a valley above its original source by gullying, mass wasting, and sheet erosion Stream Valley Development • Stream terraces – Step-like landforms found above a stream and its floodplain – Occurs when river rapidly cuts downward into its own floodplain – Represents relatively sudden change from deposition to erosion – Can be caused by rapid uplift, drops in base level, or climate changes Stream Valley Development • Incised meanders – Retain sinuous pattern as they cut vertically downward – May be produced by profound base level changes, as when rapid tectonic uplift occurs Stream Valleys on Mars • Evidence of different climate in past – Liquid water not stable on surface of Mars under present conditions • Too cold • Atmospheric pressure too low – Stream channels and terraces suggest long-term erosion by flowing water – Lack of smaller tributaries is puzzling, but these do exist for channels networks in more ancient terrains on Mars – Requires warmer, wetter Mars – NASA missions targeting such locations

Streams and Floods

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