Streams and Floods Physical Geology, Chapter 10

Chp. 10 Running Water Running water is the most important geologic agent of 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 Condensation Precipitation Transpiration Runoff Infiltration

Streams 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 - bottom of the channel Floodplain flat valley floor composed of sediment deposited by the stream

Drainage Basins Drainage basin - the total area drained by a stream and its tributaries Tributary -a small stream flowing into a larger one Can be outlined on a map by drawing a line around the region drained by all of the tributaries of a stream Divide - ridge or strip of 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 1. Dendritic - drainage pattern resembling the branches of a tree 2. Radial pattern - streams diverge outward like the spokes of a wheel Typically form on high conical mountains 3. Rectangular pattern - tributaries have frequent 90° bends and join other streams at right angles 4. Trellis pattern - parallel streams with short tributaries meeting at right angles

Stream Erosion Stream erosion and deposition are controlled mainly by a river’s velocity and discharge Stream velocity is largely controlled by stream gradient (slope), channel shape and channel roughness Stream reaches maximum velocity near center of channel Floods involve an increase in velocity and total discharge (volume of water passing a particular point in a stream over time) Higher flow velocities in a stream allow it to transport coarser sediments

Stream Erosion and Deposition 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 flow 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 1.Hydraulic action - ability of flowing water to pick up and move rock and sediment 2.Solution - dissolving of rocks (e.g., limestone) 3.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 Transportation Sediment load transported by a stream can be subdivided into bed load, suspended load, and dissolved load 1.Bed Load - large or heavy particles that travel on the streambed A.Traction load - large particles that travel along the streambed by rolling, sliding or dragging B.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

Sediment Transportation Continued 2.Suspended load - sediment that is small/light enough to remain above the stream bottom by turbulent flow for an indefinite period of time 3.Dissolved load - dissolved ions produced by chemical weathering of soluble minerals upstream

Sediment Deposition Bars - ridges of sediment (usually sand or gravel) deposited in the middle or along the sides of a stream Braided streams contain an overabundance of sediment deposited as numerous bars around which water flows in a system of highly interconnected rivulets Meandering streams flow faster along the outside of bends (meanders) and more slowly along the inside, during floods.

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. The Main channel has slightly raised banks with respect to the floodplain known as natural levees

Sediment Deposition Deltas - a body of sediment deposited at the mouth of a river when the flow velocity decreases Shape of a delta will depend on whether its growth and evolution are wave-dominated, tide-dominated, or stream-dominated

Sediment Deposition Alluvial fans - large, fan- or cone-shaped piles of sediment that usually form where a stream’s velocity decreases as it emerges from a narrow mountain canyon onto a flat plain Particularly well-developed in desert regions, such as the southwestern U.S. Larger fans show grading from large sediments nearest the mountains and finer sediments deposited progressively 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 in a flood causes flood erosion Slowing of waters as flood waters recede causes flood deposits (usually of silt or clay-sized particles) to be laid down in the floodplain

Urban flooding Flash floods 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

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 levels overtop artificially raised levees, leading to extremely rapid flooding and erosion than would have occurred otherwise Wise land-use planning, including prevention of building within 100-year floodplains, is most effective

Stream Valley Development Downcutting Process of deepening a valley by erosion of the streambed V-shaped valleys typically form from downcutting of river channel combined with mass wasting and sheet erosion of valley walls Streams cannot erode below the elevation of the mouth of the stream (the base level) Base level can be sea level, a lake, or the bottom of a closed basin (like Death Valley, CA)

Graded streams Characteristic concave-up longitudinal profile Rapids and waterfalls have been smoothed out by prior downcutting 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 terraces Incised meanders Superposed streams Step-like landforms found above a stream and its floodplain Occurs when river cuts downward into its own floodplain deposits Represents change from deposition to erosion Can be caused by rapid uplift, drops in base level, or climate changes Incised meanders Retain their sinuous pattern as they cut vertically below original level May be produced by profound base level changes, as due to rapid uplift Superposed streams Rivers superposed onto mountain range from an eroding floodplain that used to exist above it Several rivers cutting through the previously partly-buried Appalachians are examples (Potomac, Susquehanna)

Stream Valleys on Mars Evidence of different climate in past Liquid water is 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 of larger channels is puzzling, but these do exist for channels networks in more ancient terrains on Mars Requires warmer, wetter Mars in past NASA missions targeting such locations