Geology - Running Water - Quick Guides

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Study GuideGeology–Running Water1.Stream DynamicsStreams may look calm on the surface, but they are always moving and shaping the land aroundthem. How fast a stream flows and how much work it can do depend on several important factors.Let’s break them down step by step.1.1Stream Gradient: How Steep Is the Stream?Thestream gradientis the slope of the stream channel—how steeply it goes downhill.For example, if a stream has a gradient of10 feet per mile, that means the stream drops 10 feet inelevation over one mile of horizontal distance.•Gradients arehighest near the headwaters, where the stream begins.•Gradients arelowest near the mouth, where the stream empties into a larger body of water.•Thesteeper thegradient, thefaster the stream flows.Think of water flowing down a steep hill versus a gentle slope. Water always moves faster on steeperground.1.2Channel Shape and Texture: The Stream’s PathTheshapeandroughnessof the stream channel also affect how fast water flows.1.2.1Channel Shape•Anarrow, deep channelthat isV-shaped or semicircularallows water to flow faster.•Awide, shallow channelslows the water because more of it touches the stream bed,increasing friction.

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Study Guide1.2.2Channel Texture•Asmooth channelcreates less friction, so water moves faster.•A channel filled withrocks or bouldersincreases friction and slows the flow.Fastest flow happens when the channel is narrow, deep, smooth, and semicircular.1.3Stream Velocity: How Fast the Water MovesStream velocityis the speed at which water flows through the channel.•A fast river can move at about5 miles per hour.•Water movesfastest in the center of the channel, where it is deepest and has the leastfriction.•Water movesslowest along the banks and bottom, where it rubs against rocks andsediment.The faster the stream moves, the more powerful it becomes.Higher velocity means more erosionand a greater ability to carry sediment over long distances.1.4Stream Discharge: How Much Water Is Flowing?Stream dischargeis the amount of water that passes a specific point in a given amount of time.•It is usually measured incubic feet per second (ft³/s).•Discharge depends on three factors:○Channel width○Channel depth○Stream velocityDischarge FormulaDischarge = width × depth × velocity

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Study Guide1.5Changes in Discharge•Discharge usuallyincreases downstreambecause tributaries and groundwater add morewater to the stream.•Discharge can change withseasons and rainfall.•Duringflooding, discharge and velocity can increase dramatically—sometimes up to100times greater than normal.When floods occur, processes likeerosion, transportation, and depositionbecome much moreintense and reshape the landscape quickly.2.Stream ErosionStreams are one of the most powerful agents of erosion on Earth. Over long periods of time, flowingwater can carve deep valleys and dramatic landforms. A famous example is theGrand Canyon,which was shaped by running water over millions of years.As a stream flows, it does more than just cut into solid rock. It also wears down sediments already inthe channel and breaks apart the rocks and soil particles it carries. Streams erode the Earth’s surfaceinthree main ways: hydraulic action, abrasion, and solution.2.1Hydraulic AcƟon: The Power of Moving WaterHydraulic actionis the ability of flowing water to loosen and carry away rock and sediment.•Faster-moving water has more force.•Steeper stream gradients increase erosion.•Rough, uneven stream bottoms make erosion stronger.When water flows over cracks and uneven surfaces, it can “grab” rock fragments and pull them loose.Swirling currents, callededdies, help lift particles from the stream bed, making erosion even moreeffective.

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Study Guide2.2Abrasion: Rock Grinding Against RockAbrasionhappens when sediment carried by the stream scrapes and grinds against the channel bed.•Rock fragments, gravel, and sand act like sandpaper.•These particles constantly strike the stream bed, smoothing it over time.•Sediment particles also collide with each other, breaking into smaller pieces.Streams that carrymore sedimentcausemore erosion. Large, heavy particles hit the bed moreoften and with greater force than smaller particles, increasing the rate of erosion.2.3Potholes: Circular Erosion FeaturesPotholesare round, bowl-shaped holes carved into bedrock by swirling sediment.•They form where abrasive particles spin in one place.•Erosion is strongest duringfloods, when water moves faster andcarries more sediment.•Potholes are common insofter rockor where flow is squeezed into narrow spaces, such asbetween large boulders.2.4SoluƟon: Chemical Erosion in StreamsSome rocks can be eroded throughsolution, a type of chemicalweathering.•Stream water is slightly acidic.•Rocks likelimestoneand sedimentary rocks cemented withcalcitecan dissolve.•When the calcite cement dissolves, sediment particles are freed.Once loosened, these particles can be easily carried away by the stream through hydraulic action.3. Sediment LoadAs a stream flows, it carriessediment, which is known as itssediment load. Most of this material istransported eitherdissolved in the waterorsuspended within it. The rest moves along the bottomof the stream and is called thebed load.

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Study Guide3.1 Dissolved Load: Sediment in SoluƟonThedissolved loadconsists of Earth materials that have beenchemically dissolvedin water andbroken into ions.•This material usually enters the stream throughgroundwater.•Common dissolved ions include:○Calcium○Bicarbonate○Potassium○Sulfate○ChlorideAs water flows downstream, these ions canreact chemicallyto form new minerals if conditions areright. In quiet or trapped pools, minerals may alsoprecipitate out of the waterwhen evaporationoccurs.3.2 Suspended Load: Sediment FloaƟng in the WaterThesuspended loadis made up offine-grained particlesthat stay mixed within the flowing water.•During floods, rivers often lookmuddy or brownbecause of large amounts of suspendedsediment.•Suspended load mostly includessilt and clay, which are light enough to remain carried bythe current.•Most of a stream’s sediment is transported this way.Because these particles do not scrape along the stream bed,suspended load contributes verylittle to erosion.3.3 Bed Load: Sediment Moving Along the BoƩomThebed loadincludesheavier, coarse-grained materialsuch as sand, gravel, and small rocks thattravel along the bottom of the stream.

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Study GuideThere are two main ways bed load moves:•Traction–particles roll or slide along the stream bed.•Saltation–turbulent water briefly lifts particles, causing them to move forward in short jumpsbefore falling back to the bottom due to their weight.3.4 Capacity and Competence: How Much and How Big?Streams differ in how much sediment they can carry and what size particles they can move.3.4.1 Capacity•Capacityis themaximum amount of sedimenta stream can transport.•It depends ondischarge.•More water flowing in the stream meansgreater capacity.3.4.2 Competence•Competencerefers to thelargest particle sizea stream can move.•It depends onvelocity.•Faster water can carrylarger particles.Because floods greatly increase both discharge and velocity, asingle flood event can cause moreerosion than decades or even centuries of normal stream flow.4.Stream DeposiƟonAs a stream flows, its sediment isconstantly being eroded, deposited, and redeposited. Thishappens again and again, especially during changes in climate or major events like floods. Sedimentcan be depositedalong the stream’s courseorat the stream’smouth, where the water slowsdown the most.Deposits formed near the mouth—such asalluvial fansanddeltas—are usually more stablebecause they form inlower-energy environmentsthat are less affected by changes in stream flow.

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