GEOL 1340 Unit 7 Rivers and Floods

Streams as Geologic Agents

Major agents of erosion, transportation, and deposition that carve landscapes and shape the Earth's surface.

Watershed (Drainage Basin)

The total land area drained by a network of streams.

Drainage Divides

Topographic high points that separate adjacent watersheds or drainage basins.

Stream Flow Direction

Water flows downhill from its headwaters (source) to its mouth where it empties into a lake, ocean, or another river.

Permanent Streams

Streams that flow year-round because the channel lies below the water table, receiving constant groundwater discharge.

Ephemeral Streams

Streams that flow episodically or seasonally, often losing water into the subsurface through beds located above the water table.

Dendritic Drainage Pattern

A branching, tree-like drainage pattern developing on uniform, flat-lying rock or sediment.

Radial Drainage Pattern

Streams flowing outward in all directions from a central high point, such as a volcano or dome.

Trellis Drainage Pattern

Rectangular drainage patterns forming in areas of folded, alternating hard and soft sedimentary ridges and valleys.

Rectangular Drainage Pattern

Grid-like drainage patterns controlled by jointed or faulted bedrock.

Downcutting

The deepening of a stream channel into the bedrock or sediment via downward erosion.

Headward Erosion

The lengthening of a stream channel upstream as erosion cuts back into the slope at the source.

Stream Turbulence

Caused by irregular channel roughness, obstacles, high velocity, and sharp bends, creating swirling eddies.

Stream Erosion Mechanisms

Includes scouring (lifting loose sediment), abrasion (sediment-laden water wearing away solid rock), and dissolution.

Dissolved Load

Ions dissolved in the water from mineral weathering.

Suspended Load

Fine-grained silt and clay particles kept afloat by fluid turbulence.

Bed Load

Coarser sand, pebbles, and gravel that move along the channel bed via traction (sliding/rolling) or saltation (bouncing/skipping).

Sediment Deposition

Occurs when a stream's velocity and competence decrease, causing suspended and bed materials to drop out of flow.

Base Level

The lowest elevation to which a stream can downcut or erode its channel.

Ultimate Base Level

Sea level.

Local Base Level

Temporary barriers like lakes or resistant rock ledges that restrict downcutting upstream.

Base Level Changes

Raising base level induces sediment deposition upstream, while lowering base level accelerates downcutting and erosion.

Braided Rivers

A network of multiple, intertwined channels separated by shifting sand and gravel bars, formed where streams are overloaded with coarse sediment.

Meandering Rivers

Highly sinuous, looping channels found along lower-gradient, flatter portions of a river’s profile.

Cut Bank

The outside curve of a meander where water flows fastest, causing aggressive bank erosion.

Point Bar

The inside curve of a meander where water moves slowly, resulting in sediment deposition.

Oxbow Lakes

Crescent-shaped lakes formed when a meander neck is breached and abandoned during a flood.

Yazoo Streams

Tributary streams that flow parallel to the main trunk river within the floodplain because natural levees block direct entry.

Alluvial Fans

Conical, fan-shaped sediment deposits that accumulate where a steep mountain stream flows onto a flat plain.

Deltas

Sedimentary deposits formed where a stream enters standing water, dropping its sediment load into a fan of distributary channels.

Avulsion

The process where a river abandons its old channel or delta route for a new, steeper path.

Stream Terraces

Step-like benches formed when an alluvium-filled valley undergoes renewed downcutting, leaving the old floodplain elevated above the new active channel.

Causes of Flooding

Abrupt heavy rain, prolonged continuous rains saturating soil, rapid spring snowmelt, or catastrophic dam/levee failure.

Seasonal / Slow-Onset Floods

Floods that develop over hours to days, such as spring snowmelt or seasonal monsoons, allowing time for evacuation.

Flash Floods

Floods that strike rapidly with little warning, producing a destructive wall of debris-laden water driven by intense localized rainfall or dam failures.

Effects of Urbanization on Hydrographs

Replaces permeable soils with concrete, drastically shortening lag time and increasing peak flood discharge.

Annual Probability Formula

Calculated as $P = \frac{1}{\text{Recurrence Interval}}$, meaning a 100-year flood has a 1% chance each year.

Flood Control Structures

Dams, artificial levees, floodwalls, and diversion channels used to manage or bypass floodwaters.