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Geology - Running Water

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    Study Guide Geology – Running Water 1. Stream Dynamics Streams may look calm on the surface, but they are always moving and shaping the land around them. 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. 1.1 Stream Gradient: How Steep Is the Stream? The stream gradient is the slope of the stream channel — how steeply it goes downhill. For example, if a stream has a gradient of 10 feet per mile , that means the stream drops 10 feet in elevation over one mile of horizontal distance. • Gradients are highest near the headwaters , where the stream begins. • Gradients are lowest near the mouth , where the stream empties into a larger body of water. • The steeper the gradient , the faster the stream flows . Think of water flowing down a steep hill versus a gentle slope. Water always moves faster on steeper ground. 1.2 Channel Shape and Texture: The Stream’s Path The shape and roughness of the stream channel also affect how fast water flows. 1.2.1 Channel Shape • A narrow, deep channel that is V - shaped or semicircular allows water to flow faster. • A wide, shallow channel slows the water because more of it touches the stream bed, increasing friction.

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    Study Guide 1.2.2 Channel Texture • A smooth channel creates less friction, so water moves faster. • A channel filled with rocks or boulders increases friction and slows the flow. Fastest flow happens when the channel is narrow, deep, smooth, and semicircular. 1.3 Stream Velocity: How Fast the Water Moves Stream velocity is the speed at which water flows through the channel. • A fast river can move at about 5 miles per hour . • Water moves fastest in the center of the channel , where it is deepest and has the least friction. • Water moves slowest along the banks and bottom , where it rubs against rocks and sediment. The faster the stream moves, the more powerful it becomes. Higher velocity means more erosion and a greater ability to carry sediment over long distances. 1.4 Stream Discharge: How Much Water Is Flowing? Stream discharge is the amount of water that passes a specific point in a given amount of time. • It is usually measured in cubic feet per second (ft³/s) . • Discharge depends on three factors: ○ Channel width ○ Channel depth ○ Stream velocity Discharge Formula Discharge = width × depth × velocity

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    Study Guide 1.5 Changes in Discharge • Discharge usually increases downstream because tributaries and groundwater add more water to the stream. • Discharge can change with seasons and rainfall . • During flooding , discharge and velocity can increase dramatically — sometimes up to 100 times greater than normal . When floods occur, processes like erosion, transportation, and deposition become much more intense and reshape the landscape quickly. 2. Stream Erosion Streams are one of the most powerful agents of erosion on Earth. Over long periods of time, flowing water can carve deep valleys and dramatic landforms. A famous example is the Grand 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 in the channel and breaks apart the rocks and soil particles it carries. Streams erode the Earth’s surface in three main ways : hydraulic action, abrasion, and solution. 2.1 Hydraulic Ac Ɵ on: The Power of Moving Water Hydraulic action is 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, called eddies , help lift particles from the stream bed, making erosion even more effective.

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    Study Guide 2.2 Abrasion: Rock Grinding Against Rock Abrasion happens 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 carry more sediment cause more erosion . Large, heavy particles hit the bed more often and with greater force than smaller particles, increasing the rate of erosion. 2.3 Potholes: Circular Erosion Features Potholes are round, bowl - shaped holes carved into bedrock by swirling sediment. • They form where abrasive particles spin in one place. • Erosion is strongest during floods , when water moves faster and carries more sediment. • Potholes are common in softer rock or where flow is squeezed into narrow spaces, such as between large boulders. 2.4 Solu Ɵ on: Chemical Erosion in Streams Some rocks can be eroded through solution , a type of chemical weathering. • Stream water is slightly acidic. • Rocks like limestone and sedimentary rocks cemented with calcite can 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 Load As a stream flows, it carries sediment , which is known as its sediment load . Most of this material is transported either dissolved in the water or suspended within it . The rest moves along the bottom of the stream and is called the bed load .

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    Study Guide 3.1 Dissolved Load: Sediment in Solu Ɵ on The dissolved load consists of Earth materials that have been chemically dissolved in water and broken into ions. • This material usually enters the stream through groundwater . • Common dissolved ions include: ○ Calcium ○ Bicarbonate ○ Potassium ○ Sulfate ○ Chloride As water flows downstream, these ions can react chemically to form new minerals if conditions are right. In quiet or trapped pools, minerals may also precipitate out of the water when evaporation occurs. 3.2 Suspended Load: Sediment Floa Ɵ ng in the Water The suspended load is made up of fine - grained particles that stay mixed within the flowing water. • During floods, rivers often look muddy or brown because of large amounts of suspended sediment. • Suspended load mostly includes silt and clay , which are light enough to remain carried by the current. • Most of a stream’s sediment is transported this way. Because these particles do not scrape along the stream bed, suspended load contributes very little to erosion . 3.3 Bed Load: Sediment Moving Along the Bo Ʃ om The bed load includes heavier, coarse - grained material such as sand, gravel, and small rocks that travel along the bottom of the stream.

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    Study Guide There 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 jumps before 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 • Capacity is the maximum amount of sediment a stream can transport. • It depends on discharge . • More water flowing in the stream means greater capacity . 3.4.2 Competence • Competence refers to the largest particle size a stream can move. • It depends on velocity . • Faster water can carry larger particles . Because floods greatly increase both discharge and velocity, a single flood event can cause more erosion than decades or even centuries of normal stream flow . 4. Stream Deposi Ɵ on As a stream flows, its sediment is constantly being eroded, deposited, and redeposited . This happens again and again, especially during changes in climate or major events like floods. Sediment can be deposited along the stream’s course or at the stream’s mouth , where the water slows down the most. Deposits formed near the mouth — such as alluvial fans and deltas — are usually more stable because they form in lower - energy environments that are less affected by changes in stream flow.

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