Running Water

Running Water Includes all water that falls on Earth as rain or snow, and moves downhill Running water weathers the rock and erosion then follows Bedrock is attacked by water in two ways: Physically wears down the rock Suspended material pounds and breaks up the rock as it moves over it, called saltation

Running Water Abrasion is the grinding action by sand, pebbles and boulders. Rapidly moving water removes loose fragments (aiding in erosion) Chemical action occurs as water dissolves minerals in the bedrock

Removal of Weathered Rock Water removes weathered rock in 3 forms: Dissolved minerals in solution Slightly larger particles will float within the moving water, in suspension (silt, clay) Bedload are those materials at the bottom of the water flow, which are too heavy to float along (sand, gravel, pebbles, boulders)

Carrying Power and Load Carrying power of a stream is indicated by the total amount of sediment and the size of particles being moved by it. A stream’s carrying power depends on: Its speed (or velocity) Its volume (or discharge rate) The time of year

The Life Cycle of a River Ancient rivers leave evidence of their existence by a “V” shaped valley and the presence of rounded rock fragments There are three basic stages of a river Young (immature) Stage Medium (intermediate) Stage Mature Stage

Young Stage River erodes upper valley walls first (they are the first exposed to weathering) Young rivers are characterized by steep walls (canyons) in a “V” cross-section.

Middle Stage River has reached the point where it no longer weathers the rock below (reaches its base level) Erosion now begins to work outward on valley walls, and the river begins to widen, becoming shallower and slower

Mature Stage (Old Age) The river begins to “meander” The valley becomes wide and flat It often re-routes itself and cuts off a meander to form an isolated oxbow lake

River Stages

Flood Plains Mature rivers may overflow during heavy run-off The land flooded beside it is called the flood plain If the river erodes the flood plains at different rates, a series of steps or terraces may form.

Stream Erosion and Deposition In a stream with a bend: velocity is greatest at the outer edge of the stream bend and greater erosion occurs here velocity is slower at the inside of the bend, resulting in reduced erosion, and in fact deposition can occur

Lengthening the River Valley A single heavy rain can form a miniature stream valley (V-shaped and have tributaries running into it) When the rain ends the stream may disappear, but the small valley remains. This is called a gully Gullies grow in length and width every time it rains. Headward erosion makes the gully longer If the stream cuts below the water table, it will become a permanent stream.

A Gully

Drainage Basins and Divides High land that separates one gully from the next is called a divide A river and all of its tributaries is called a river system The drainage basin (watershed) of a river includes all the land that drains into the river (directly or indirectly)

Four Types of Drainage Basins

Stream Piracy Occurs when a stream erodes by headward erosion until it captures another stream. Also called stream capture

Water and Wind Gaps If water meets a region of resistant rock that erodes more slowly than the rock around it, a narrow gap (water gap) occurs Once water has dried up, a dry narrow gap (now called a wind gap) remains.

Potholes and Plunge Pools Potholes occur when boulders, pebbles and sand swirl around and grind out an oval or circular basin When potholes are very large, they are called plunge pools This can cause the recession of a waterfall due to erosion. Niagara Falls

DEPOSITION OF RIVER SEDIMENTS Deposition occurs wherever velocity decreases due to: Decreased slope Widening of the river Meeting with an obstruction (outcrop, curve etc) Areas of increased evaporation Increased seepage Diversion for irrigation or water supply Where rivers meet large bodies of water

Deltas A delta forms when the river flows into a quiet body of water (lake, gulf or inland sea), because the river slows down Most of the sediment drops at the river’s mouth, with larger materials being dropped first Deltas rarely form where a river meets an ocean because currents/waves are too strong and carry sediment away

Deltas

Alluvial Fans A fan shaped deposit formed where a stream ends, usually at the bottom of a steep hill where it meets flat land Forms on land, not in the water Composed of sands and gravels Surface is sloping and not flat like a delta

Sediments on Flood Plains When a river overflows its banks, it slows down, dropping suspended materials (sediments) More sediment is dropped at the banks, forming thick deposits which build up, called levees Fertility of the flood plain (low areas behind the levees) is increased due to nutrients and materials deposited during flooding

Causes of River Flooding Flash floods: a large volume of rain over a short period Abnormal period of excessive rain Natural dam where the river is obstructed (ice dam, volcanic flow, landslides) Human effects like clear cut logging

Preventing Floods Flooding can be prevented by: Planting trees to help rain absorption Dams to control river flow Artificial levees (eg. sandbags) to raise banks Spillway channels to take the overflow and re-route it