Yellowstone River braided near its base level The Base level – the river mouth • Base level is the lowest elevation at which the running water can flow, erode and transport load. – @ base level the stream deposits all the sediments it carries • The ultimate base level is the sea level • A relative base level can be a lake, or a dam. Burke Lake is formed by a dam on South Run, a tributary stream of the Potomac River Evolution of the Mississippi delta during the past 6000 years The works of the River • Water erodes, transports and deposits Hijulstrom diagram • From the headwaters to the mouth/base level the river works at shaping the landscape by erosion, transport and deposition EROSION rivers erode by Abrasion (mechanical): the river cuts its path • Stronger currents lift particles more effectively. Dissolution (chemical erosion) Stream erosion The stream channel (→ valley) is an erosional feature The Grand Canyon was eroded by the Colorado River Erosion by water Potholes Stream erosion on rocks Great Falls NP TRANSPORT • LOAD = the stream’s transported sediment. • 3 types of load: dissolved, suspended, bed load • Capacity is the maximum load a stream can transport. • Competence refers to the size of the material transported Suspended load Suspended load makes the water look discolored Mississippi Delta Bed load Larger, heavier fragments are dragged along the bottom of the stream DEPOSITION • When stream velocity decreases competence (size of load) is reduced → sediment drops down in a well sorted sequence bar is an elevated region of sediment (such as sand or gravel) that has been deposited by the flow. Mid channel bar Point bar Point bar forms in the inner side of the channel Alluvial fans • Some streams have water only during certain times of the years, the delta forms on land on a plain instead on into a body of water • Alluvial fans called bajadas form at the mouth of canyons from the debris transported by such streams – They are a form of mass wasting! Alluvial fan • Some rivers drain in a large plain, like the Okavango delta in Botswana, which has water flowing at the end of the rain season Streams and sea level change • The stream profile is adjusted to the base level • If the sea level changes the patterns of erosion-transportdeposition will change • If the sea level changes the base level changes → the stream starts again to erode downward to reach the new base level • Meanders become narrower Stream terraces form Stream/River Terraces • Remnants of a former floodplain • River has adjusted to a relative drop in base level by down-cutting. The US capitol is built on a river terrace How many river related features can you identify in this image? Flood • When discharge is more than channel manage the river goes into flood • The floodplain is created by the river to manage the discharge • Flood is the natural way in which the river manages the excess of discharge sent by the water cycle! The Natural Levee of a River Levees are elongate ridges of mud and/or silt that form on the river floodplains immediately adjacent to the river banks. The levee grows from repeated floods The benefits of floods • Floods bring the load of the river onto the land, making it fertile. Agriculture develops most succesfully in Floodplains because of the nutrient-rich soil • Egypt civilization was built on the fertile flood plain of the Nile River he Nile River and its corridor of agriculture contrast sharply with the arid surroundings of Egypt. Types of flood: Regional Flood • Seasonal: it is controlled by – precipitation intensity and duration – rate of melting of snow – ground saturation • How it develops: – Slow to build up (days to weeks), slow to recede (weeks to months!) • It covers large areas (regions) The Red River, ND plains The flatness and slope of the Red River Valley contribute to its susceptibility to flooding. Red River @ Fargo record Spring floods The hydrograph is a graphic representation of a river’s discharge over time ~1 week ~1 month Today’s values The Mississippi The Mississippi Types of flood: Flash Flood • Limited areal extent – Fast occurring • Depends on: – Amount + intensity of precipitation – Surface conditions • Steep slopes – mountainous areas • Urbanized ground Case study: Big Thompson Canyon CO • Big Thompson River is a youthful stream with headwaters in the Rock Mountains Canyon topography Big Thompson drainage basin 1000ft • 1976 flash flood • 2013 Flood footage Other occurrence of flash flood Flash flood can happen also • on floodplains were where farmland or urban development has quickly replaced natural vegetation there is increases soil erosion and stream deposition • In highly urbanized areas, where cement cover prevents water from being absorbed in the ground Constitution Ave. And Canal Road, DC July 2019 Urbanization and flash floods Increases impervious cover the ground cannot absorb water Reduces the lag time of flood (time precipitation to flood) Urban lag time Mitigation of flood hazards: Monitoring Mitigation by engineering: Flood control Structures Louisiana • Engineering efforts • Artificial levees • Flood-control dams • Channelization Arroyo Seco, Los Angeles Lake Braddock Dam, VA Flood hazards The Mississippi, the US Congress and the US Army Corps of Engineers Clearing navigation obstacles and preventing floods since 1826 by • Removal of sandbars • Dredging • Locks and Dams • “Levees only” solution (after 1861 ) • Fairfax Co. has many flood control dams, the closest to campus are lake Royal and lake Braddock. Burke lake is the largest flood control dam in Fairfax Recurrence of floods: the 100-yr flood plain • there is a 1-in-100 chance that a flood this size will happen during any given year. • Basis for the National Flood Insurance Program (not forecast!) – Insurance as a way to mitigate the effect of a disaster Mitigation by land use planning: Flood plain management • Choosing to develop land based on the natural surface process occurrence and intensity: • Positioning farming and recreational in the flood plain – Residential and more critical infrastructures on river terraces Mitigation of flood hazards: Monitoring Mitigation by engineering: Flood control Structures Louisiana • Engineering efforts • Artificial levees • Flood-control dams • Channelization Arroyo Seco, Los Angeles Lake Braddock Dam, VA Flood hazards The Mississippi, the US Congress and the US Army Corps of Engineers Clearing navigation obstacles and preventing floods since 1826 by • Removal of sandbars • Dredging • Locks and Dams • “Levees only” solution (after 1861 ) • Fairfax Co. has many flood control dams, the closest to campus are lake Royal and lake Braddock. Burke lake is the largest flood control dam in Fairfax Recurrence of floods: the 100-yr flood plain • there is a 1-in-100 chance that a flood this size will happen during any given year. • Basis for the National Flood Insurance Program (not forecast!) – Insurance as a way to mitigate the effect of a disaster Mitigation by land use planning: Flood plain management • Choosing to develop land based on the natural surface process occurrence and intensity: • Positioning farming and recreational in the flood plain – Residential and more critical infrastructures on river terraces Groundwater What is groundwater • • • Groundwater is part of the hydrologic cycle comes from precipitation and runoff and is absorbed by the ground. Groundwater flows inside the ground Where is the groundwater The water table is the upper limit of the zone of saturation An aquifer is the portion of the underground sediments or rock that is filled with groundwater GROUNDWATER All pore spaces are filled completely with water The Water Table and Aquifer • The water table is the upper limit of the zone of saturation • It is not a flat surface: there are high areas and low areas just like the hills and valleys found on land. – Its depth varies seasonably and yearly Symbol used to mark the water table Aquifer Porosity • Porosity: Percent of volume that is void space. – Sediment: Determined by how tightly packed and how clean (silt and clay), (usually between 20 and 40%) – Rock: Determined by size and number of fractures (most often very low, 1 mile, up to 3miles in some places • It accumulated over millions of years through snowfall. • It contains 90% of the ice on Earth A system of rivers and lakes (400+) have been identified using geophysical methods The largest under ice lake in Anatarctica

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