Geology 18 (Rivers and Springs)

Earth and Space Sciences X

29 Oct 201543:05

EducationalLearning

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TLDRThe lecture discusses surface water, focusing on rivers and their features like flow velocity, erosion, sediment transport, and deposition. It covers concepts including drainage basins, continental divides, longitudinal profiles, laminar versus turbulent flow, and bedrock versus alluvial channels. Other key topics are meandering streams, oxbow lakes, braided channels, base level, downcutting, v-shaped valleys, deltas, distributaries, and alluvial fans. The goal is providing an overview of how rivers work by cutting down to base level while transporting sediment from mountainous headwaters to coastal mouths.

Takeaways

😃 Rivers are everywhere, from major islands to famous rivers like the Amazon and Nile
😊 Only 2.5% of earth's water is freshwater, most is locked in glaciers and ice
🚰 Rivers drain rainfall and snowmelt, carving valleys and carrying sediment
🌊 Stream flow velocity depends on slope, channel shape and roughness
🏞 Longitudinal stream profiles are concave, with steepest gradients upstream
⚙️ Rivers erode and transport sediment in dissolved, suspended and bed loads
🛤 Meandering streams migrate slowly, forming oxbow lakes when bends join
🌧 Changing base level causes adjustments in stream erosion and deposition
🔧 Tectonic uplift steepens valleys while subsidence leads to alluvial fans
💧 Deltas form when sediment-laden rivers reach lakes and oceans

Q & A

What are the three main zones that river systems can be divided into?
-The three main zones that river systems can be divided into are: 1) The headwaters - the source of the river located in the drainage basin where sediment production occurs; 2) The trunk stream - the central zone where sediment is transported but neither eroded nor deposited; and 3) The mouth - the downstream point where the river empties into a larger body of water and sediment deposition occurs.
How does discharge relate to flow velocity and channel size?
-As discharge increases, flow velocity and channel size also increase. Higher discharge means more water flowing through the channel, which increases the cross-sectional area to wetted perimeter ratio, allowing the water to flow more efficiently and quickly while enlarging the channel.
What factors control whether water will infiltrate into the ground or run off on the surface?
-Factors that control infiltration vs. surface runoff include: rainfall intensity and duration, pre-existing soil moisture levels, soil type (water infiltrates more easily into sand than clay), topography/slope of land (steeper = more runoff), and vegetative cover (more vegetation = more infiltration).
What are the three main ways that streams transport sediment?
-The three main ways streams transport sediment are: 1) Dissolved load - dissolved minerals that travel invisibly in the water solution; 2) Suspended load - silt, clay, and fine particles that float in the water column; and 3) Bed load - sand, gravel, cobbles, and boulders that slide, roll, or saltate (bounce) along the stream bed.
How do meanders and oxbow lakes form?
-As a meandering stream erodes the outer banks of its bends (cut banks), the bends migrate over time. Eventually the adjacent loops can connect, cutting off the meander bend and abandoning an oxbow lake in the previous floodplain.
What happens when base level is changed due to factors like tectonic activity or dam installation?
-When base level rises due to a factor like a newly installed dam, deposition occurs as the stream attempts to re-establish its grade. When base level falls, such as during tectonic uplift, the stream will erode downwards more intensely in an attempt to reach its original base level.
Where are coarse vs. fine sediments deposited in a delta?
-In a delta, coarse sediments like gravel are deposited near the river mouth where flow velocity rapidly drops off. Finer silt and clay particles stay in suspension longer and are carried out to be deposited along the outer fringe of the delta.
How do factors like climate and human activity influence the character of a river system?
-Climate influences water availability, which impacts stream discharge and therefore channel dimensions and sediment transport capacity. Human interventions like dam building or deforestation also greatly impact discharge patterns, sediment load, and more.
What is the difference between a bedrock channel and an alluvial channel?
-A bedrock channel is carved directly into the underlying bedrock geology, while an alluvial channel forms from previously deposited sediments, allowing it to meander across a floodplain as sediment is eroded and redeposited over time.
Why do braided streams have so many converging and diverging channels?
-Braided streams form where there is a large bed load of coarse, erodible sediment and unstable banks. The many shifting channels enable efficient transport of the abundant loose sediment.

Outlines

00:00

🏞️ Introducing Surface Water and Rivers

The first paragraph introduces the topic of surface water, focusing specifically on rivers. It discusses how rivers are ubiquitous, found across continents and islands worldwide. Examples are provided, like the famous Amazon and Nile Rivers. An image is shown of the serene Mississippi River to set the stage.

05:02

🌊 Stream Formation and Classification

The second paragraph delves into how streams and rivers form. It covers concepts like sheet flow, rills, gullies, brooks, and defines key terminology. There is discussion on size thresholds, like how a river must have substantial flow and many tributaries.

10:03

🚣 Sediment Transport in Rivers

The third paragraph focuses on sediment transport. It discusses the origin of sediment in headwaters regions and its eventual deposition at the river mouth, usually a delta. Key principles are introduced like channels being depositional, erosional, or neutral.

15:04

💦 Factors Influencing River Velocity

Paragraph four examines factors that control flow velocity in rivers. Concepts covered include channel gradient, shape, roughness, size, and obstacles. Comparisons are made between turbulent and laminar flows.

20:05

🛶 Stream Processes and Erosion

A variety of fluvial processes and types of erosion are overviewed next. This includes abrasion, corrosion, hydraulic action, and more. Sorting of sediment is also touched on based on particle size.

25:06

🚢 Sediment Transport Mechanisms

The ways sediment is moved in rivers is covered here - dissolved load, suspended load, and bed load. The differences between saltation, sliding, and rolling are provided as examples.

30:07

🪨 River Channels: Bedrock vs. Alluvial

Paragraph seven makes key distinctions between bedrock river channels and alluvial ones. Bedrock channels cut into solid rock while alluvial channels shift through loose sediment. Images and examples are shown.

35:07

🌊 River Profiles and Base Level Concepts

Next, base level and graded streams are introduced to explain concave river profiles. The ultimate base level of sea level is discussed, along with how uplift and dams can perturb this balance.

40:09

👣 Impact of Rivers on Landscapes

Finally, paragraph nine explores the impact rivers have on landscapes via erosion and deposition. Concepts covered include downcutting, V-shaped valleys, deltas, alluvial fans, and more.

Mindmap

Keywords

💡Rivers

Rivers are channels of flowing surface water that drain land areas. As discussed in the video, rivers are a key part of the earth's hydrologic cycle, carrying water from land back to the oceans. Examples from the script include 'the oceans the rivers the ponds', 'rivers are everywhere in the world', and 'major rivers like the Amazon and the Nile'.

💡Surface water

Surface water refers to water features on the earth's surface, such as rivers, lakes, streams, etc. As stated in the intro, the lecture focuses specifically on surface water: 'surface water today surface water is a really cool subject to talk about because we deal with it all the time'.

💡Hydrologic cycle

The hydrologic or water cycle describes the continuous movement of water between land, atmosphere and oceans. It involves processes like evaporation, precipitation, runoff and groundwater flow. The video visualizes this cycle showing 'evaporation', 'precipitation' and water 'infiltrate down into the ground'.

💡Runoff

Runoff is the water from rain, snowmelt, or other sources that flows over the land surface, eventually reaching streams, rivers, lakes or oceans. As discussed in the video, factors like soil type, slope and vegetation cover influence runoff.

💡Channel

A channel is a physical landform consisting of a narrow trough cut into the land for conveying flowing water. Examples in the script include 'the channel bed', 'bedrock channels', and 'alluvial channels' that form in previously deposited sediment.

💡Discharge

Discharge refers to the volume of water moving through a river channel over time. It increases predictably as river width, depth and flow velocity increase downstream towards the river mouth.

💡Base level

The base level concept refers to the lowest point to which a river can erode its channel. It is ultimately sea level, but can be changed locally by features like lakes or resistant rock layers. This drives long term erosion and landscape evolution.

💡Meanders

Meanders are looping, sinuous bends in a river channel. Due to erosion and deposition processes, meander loops often migrate over time, sometimes getting cut off to form oxbow lakes.

💡Braided channels

These are rivers characterized by multiple, small intertwined channels that continually branch and rejoin around gravel bars and small islands. They form where banks are easily eroded, such as the 'loose sediment' described in the video script.

💡Deltas

River deltas are depositional landforms created where sediment-laden rivers enter lakes or oceans, decreasing flow velocity and dropping coarser sediments nearer the river mouth.

Highlights

Rivers drain much of the land area and transport water, sediment, and dissolved materials to the oceans

Rivers have distinct zones - headwaters with erosion, trunk streams with transport, and mouths with deposition

River flow velocity depends on slope, channel shape and roughness - steep, smooth, narrow rivers flow fastest

Streams carry sediment loads dissolved, suspended, or as bedload that saltates, rolls or slides

Erosional processes like abrasion and corrosion shape the landscape and transport sediment downstream

Channel patterns like dendritic, radial, rectangular or trellis reflect underlying geology and structure

Meandering streams erode cut banks and deposit point bars, sometimes getting cut off into oxbow lakes

Braided channels have dynamic, shifting networks of channels surrounding gravel bars

Base level drives erosion and deposition - lowering it causes erosion, raising it causes deposition

Uplift rejuvenates rivers, causing renewed downcutting and formation of rapids, waterfalls and V-shaped valleys

Oversupply of sediment at river mouths leads to deposition and formation of deltas or alluvial fans

The total global water is mostly (96.5%) saline ocean water. Only 2.5% is fresh water.

Stream discharge, width, depth and velocity all increase downstream towards the river mouth

Sediments deposit when stream velocity drops below their settling velocity, with coarse sediments first

The Grand Canyon displays dramatic downcutting driven by ongoing regional uplift of over a mile

Transcripts

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