Chapter 16 Erosion Overview Erosion Deposition Streams Stream

Chapter 16 Erosion

Overview • • • Erosion & Deposition Streams & Stream Systems Valley Formation Waterfalls Floodplains

Erosion • Erosion is the process by which water dislodges, dissolves, or removes surface materials. • Can happen in both overland flow or by streamflow • Stream Load – weathered material carried away by streams • Abrasion – when rocks/sediment erode a streambed – 3 key components in the erosion process • Transport – the movement of earth materials from one location to another • Deposition – the process of sediment buildup as a result of trasport • Alluvium – general term for sand, silt, and clay that have been deposited by running water; can also include larger rock material Erosion Factors

Deposition • Alluvium deposits happen downstream of a stream/river origin. – Larger materials are deposited first, further upstream • Larger materials can be broken down over time – Smaller deposits can be seen at the rive mouth • Order of deposited material is due to flow speed – Develop strata due to episodes of periodic flooding Erosion & Deposition Example

Perennial & Intermittent Streams • Perennial streams flow throughout the year – Can be found in humid regions • Colorado River • Intermittent streams flow only for part of the year – Can be found in humid as well as dry regions; dependent upon local water sources and local landscape features • Ephemeral streams only carry water immediately after a rain – Common in deserts Intermittent Streams

• Overland flow – the unchanneled downslope movement of water • Streamflow – channeled movement of water along a valley bottom Rills – small downhill grooves in which water runs Gullies – develop after rills have deepened into the hillside *Over time these can develop into streams

• Drainage Basin/Watershed – basic geomorphic unit of a river system – Distinguished by surrounding ridges & highlands that for divides • Drainage Divide – line of separation between runoff that descends in the direction of one drainage basin

Drainage Patterns

North American Drainage Basins

Stream Competence & Capacity • Competence – a stream’s ability to move particles of a specific size – Function of stream velocity • Capacity – total possible load that a stream can transport • Discharge – a stream’s volume of flow per unit of time

What can we say about the capacity and discharge of the two streams pictured above?

Stream Channel Patterns • Braided Stream– a maze of interconnected channels loaded with excess sediments – Occurs when there is a reduction in discharge • Increase in sediment load results in greater deposition rates

• Meandering Channels – intricate pattern of smooth curves – Follows a serpentine course – Typically occur when the land is flat – Outer curves experience the greatest amount of erosion • Known as the undercut bank – Inner curves experience deposition of material • Known as the point bar – Over time, an Oxbow Lake can form – Ultimate effect is valley widening

• Stream gradient affected by tectonic uplift can result in a change of base level. – Existing meanders can return to downcutting – New feature is called an entrenched meander • Stream is considered to be rejuvenated once it returns to downcutting

Meanders

Oxbow Lake

Valley Formation • Valleys are formed as a result of downcutting. – Occurs with rapid speed and/or relatively large volume. – Common closer to the headwaters of a stream • Steeper slope • Narrow valley near headwaters

Limits to Erosion • Base Level – the lowest level to which landforms can be eroded – Once eroded to this level, downslope still exists • Water flow to sea will still occur • Lakes can serve as temporary base levels for stream tributaries

Stream Gradient • Decline in elevation from headwaters to stream mouth • Graded Streams – condition in which channels adjust their slope, size, and shape so that a stream has just enough energy to transport sediment – This means a balance between erosion, transportation, and deposition