4.1.1 rivers - student

OnRamps: Rivers

Overview

• Experience College Before College

Learning Objectives

• Understand how rivers evolve:

  • Modify over time

  • Alter with land slope

  • Change along their length

• Identify river features:

  • Meander

  • Cutbank

  • Channel

  • Flood plain

  • Point Bar

Function of Rivers

• Primary role: Transport water from land to oceans; part of the hydrologic cycle.

• Driving force: Gravity causes water to flow downhill toward the ocean.

• Sediment transport: Rivers carry sediment and other materials.

• Drainage basin: Each river/stream has a specific area from which it collects water.

Drainage Basins

• Definition: The entire upstream area that drains into a specific river or stream.

• Tributaries: Smaller rivers/streams merging into a larger river on its way to the ocean.

River Dynamics

Stability

• Divides: Segregate individual drainage basins; generally stable.

• Movement: Rivers may slightly shift within basins but seldom cross divides.

Local Exploration

• Activity: Identify your nearest river and the watershed you inhabit by referencing a Texas Rivers map.

River Terminology

• Gradient: Slope of the land.

• Discharge: Water volume in cubic meters per second (m³/sec).

• Velocity: River speed in meters per second (m/sec).

• Headwaters: Starting point of a river.

• Base: River endpoint, typically sea level.

Relationship of Discharge and Velocity

• Connection: Discharge and velocity are related, with an equation required to explain their relationship, involving an additional variable.

Effects of Channel Construction

• Stream channel narrowing:

  • If depth and discharge remain constant, speed (velocity) of flow changes:

  • a. Velocity will increase

  • b. Velocity will decrease

  • c. Velocity will not change

Sediment Transport Mechanics

• Observation: Standing in a flowing river reveals force exerted on objects (e.g., sediment, plants, fish).

• Sediment behavior:

  • Fine sediments are suspended and always in motion.

  • Larger sediments shift through rolling or hopping (saltation).

  • Very large stones move only during substantial floods.

Sediment Size and River Velocity

• Graph representation: Shows sediment size transportable by rivers of varying velocities:

  • Clay and silt

  • Sand

  • Gravel and pebbles

  • Boulders

River Morphology

Influencing Factors

• Key variables:

  • Discharge

  • Velocity

  • Sediment load

  • Vegetation

  • Underlying rock type

Changes Along the River

• Profile characteristics:

  • Upper River: High gradient, straighter, narrower channel

  • Lower River: Low gradient, wider channel, flood plain, and pronounced bends.

Stream Velocity Control Variables

• Question: Select the variables controlling stream velocity:

  • a. Gradient and stream load

  • b. Discharge and sediment load

  • c. Gradient and discharge

River Morphology Types

Braided Rivers

• Conditions: Coarse sediment size (gravel/larger), abundant sediment supply, usually in eroded landscapes like mountains.

Cascade Rivers

• Characteristics: Steep gradient, proximity to source, and hard ground with large rocks.

Meandering Rivers

• Formation: Occurs with fine sediment (sand/clay), low gradient, and established floodplains (most common near the base).

Erosional and Depositional Features

• Observation Task: Identify features along a meandering river.

Meander Bend Dynamics

• Inside vs. Outside Bend:

  • Inside: Slower flow due to friction, leading to deposition.

  • Outside: Erosion occurs due to faster flow, focused energy on the outer bank.

Oxbow Lake Formation

• Processes: Erosion and deposition around bends can lead to the development of oxbow lakes.

Floodplain Development

• Carving Mechanism: A meandering river shapes its floodplain over time.

Engineering Considerations

• Question: Stream gradient after diversion compared to meander bend:

  • a. Greater

  • b. Decrease

  • c. Stay the same

Discharge Calculation Task

• Consideration: Find the discharge of a rectangular channel with specified dimensions and flow rate (with possible answers).

Confluence Analysis

• Statements About Stream C: Evaluate flow velocity, discharge, and gradient compared to streams A and B.

Erosion Impact on Stream Channels

• Question Post-Erosion: Predict depth/velocity changes and select the correct statement (four options provided).

Review Activity

• Task: Sketch and label differences between braided and meandering streams, including erosional and depositional features.