Surface Water

Streams and Rivers Over 50% of the water that falls to Earth ends up in a stream or river. Streams and rivers account for most of the erosion at the Earth’s surface.

A River System consists of a river with all of its tributaries. Tributary: A stream that runs into another stream or river. A river’s drainage basin or watershed includes all the land that drains into the river either directly or through its tributaries.

Four Factors Affect How a River Erodes and/or Transports Sediment: 1: Velocity: The distance water travels in a given amount of time. Velocity is affected by three factors: Slope of the riverbed Amount of water Shape of the riverbed

2. Gradient: The steepness of the slope of a stream or river.

4. Channel: The path or physical depression the water flows through. The size and shape of a channel affect its velocity. 3. Discharge: The amount of water that passes a certain point in a given amount of time. Discharge fluctuates depending on where it is measured [i.e. above or below tributaries] and the time of year.

Erosion and Deposition As streams flow, they erode rocks and soil, transport these sediments over long distances, and deposit them in areas of quieter water. Fast-moving water lifts and moves apart rock fragments as abrasive particles carried by the water cut away at bedrock.

Erosion: Occurs when running water abrades its bed and walls using sand, pebbles and boulders from its bed load. Forms features, such as this pothole formed in the Susquehana River in Pennsylvania

Transport: Materials are carried three ways: Solution: Dissolved particles Suspension: Particles are kept from sinking by turbulence. Bed load: Carried along the river bed (bouncing/rolling)

A stream’s ability to transport materials is classified two ways: Competence: The maximum size of particles a stream can carry Capacity: The total amount of sediment a stream can carry.

Types of load: 1. Suspended load - finer sediment (usually clay and silt) suspended by turbulence in the flow (does not contact bed). 2. Solution load - dissolved rock carried in the flow 3. Bed load - coarser sediment (sand + gravel) that slides, rolls or skips along the stream bed

Bed Load Diagram:

Deposition: Materials transported by rivers and streams are deposited when either their velocity or discharge changes. Beaches and sandbars can occur in streams and rivers as well as in oceans. Deltas and alluvial fans are excellent examples of deposition by rivers and streams.

Delta: Fan shaped deposit that forms when a river flows into a quiet or large body of water. Forms of fine sediments in water

Alluvial Fan: Forms when a steep mountain stream drains into dry level land at the base of the mountain. Forms of coarse sediments on dry land

O.K., so far we’ve learned. . . A river system consists of a river and all its tributaries, which are found within the river’s watershed/drainage basin. A river’s velocity, gradient, discharge and channel shape affect erosion. Rivers transport material in suspension, solution and bed load. The competence [size] and capacity [amount] of material carried by a stream/river is dependent upon the river’s discharge and velocity. Depositional features are formed when discharge or velocity slows, depositing the load in quieter waters.

River Valleys The continuous erosion caused by running water tends to form V-shaped valleys that grow longer and wider over time.

Canyon formation: Canyons form when a river cuts into its bed rapidly or when rock materials on the sides of the valley are resistant to erosion.

Rapids and Waterfalls: If the riverbed is steep enough it can cause white-water rapids or waterfalls. Rapids and waterfalls are temporary features because stream erosion is greatest at these points.

Floods and Floodplains River floods are naturally occurring events that sometimes threaten populations

Flood: Flash Flood: When a stream or river overflows its bank Caused by single cloudbursts in the narrow valley of a developing stream

Features of a flood plain Meander: When a river winds back and forth within its flood plain. Formed from increased erosion along river banks. Oxbow lakes: Form when a meander is cut off from the main river, usually by flooding. Levee: Elevated ridges along the banks of rivers formed from deposition of sediment.

Meander Oxbow Lake

Final summary . . . Youthful rivers form steep sided canyons and V-shaped valleys by erosion. Floods occur when a river exceeds its banks and fills a flood plain. A flash flood occurs when an individual cloudburst releases heavy rains over the narrow valley of a young stream. Meanders, oxbow lakes and natural levees are all features of a floodplain.

The End!!