2 Running WaterIncludes all water that falls on Earth as rain or snow, and moves downhillRunning water weathers the rock and erosion then followsBedrock is attacked by water in two ways:Physically wears down the rockSuspended material pounds and breaks up the rock as it moves over it, called saltation
3 Running WaterAbrasion 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
4 Removal of Weathered Rock Water removes weathered rock in 3 forms:Dissolved minerals in solutionSlightly 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)
5 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
6 The Life Cycle of a River Ancient rivers leave evidence of their existence by a “V” shaped valley and the presence of rounded rock fragmentsThere are three basic stages of a riverYoung (immature) StageMedium (intermediate) StageMature Stage
7 Young StageRiver 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.
8 Middle StageRiver 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
9 Mature Stage (Old Age) The river begins to “meander” The valley becomes wide and flatIt often re-routes itself and cuts off a meander to form an isolated oxbow lake
11 Flood Plains Mature rivers may overflow during heavy run-off The land flooded beside it is called the flood plainIf the river erodes the flood plains at different rates, a series of steps or terraces may form.
12 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 herevelocity is slower at the inside of the bend, resulting in reduced erosion, and in fact deposition can occur
13 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 gullyGullies grow in length and width every time it rains. Headward erosion makes the gully longerIf the stream cuts below the water table, it will become a permanent stream.
15 Drainage Basins and Divides High land that separates one gully from the next is called a divideA river and all of its tributaries is called a river systemThe drainage basin (watershed) of a river includes all the land that drains into the river (directly or indirectly)
18 Stream PiracyOccurs when a stream erodes by headward erosion until it captures another stream.Also called stream capture
19 Water and Wind GapsIf water meets a region of resistant rock that erodes more slowly than the rock around it, a narrow gap (water gap) occursOnce water has dried up, a dry narrow gap (now called a wind gap) remains.
20 Potholes and Plunge Pools Potholes occur when boulders, pebbles and sand swirl around and grind out an oval or circular basinWhen potholes are very large, they are called plunge poolsThis can cause the recession of a waterfall due to erosion.Niagara Falls
21 DEPOSITION OF RIVER SEDIMENTS Deposition occurs wherever velocity decreases due to:Decreased slopeWidening of the riverMeeting with an obstruction (outcrop, curve etc)Areas of increased evaporationIncreased seepageDiversion for irrigation or water supplyWhere rivers meet large bodies of water
22 DeltasA delta forms when the river flows into a quiet body of water (lake, gulf or inland sea), because the river slows downMost of the sediment drops at the river’s mouth, with larger materials being dropped firstDeltas rarely form where a river meets an ocean because currents/waves are too strong and carry sediment away
24 Alluvial FansA fan shaped deposit formed where a stream ends, usually at the bottom of a steep hill where it meets flat landForms on land, not in the waterComposed of sands and gravelsSurface is sloping and not flat like a delta
25 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 leveesFertility of the flood plain (low areas behind the levees) is increased due to nutrients and materials deposited during flooding
27 Causes of River Flooding Flash floods: a large volume of rain over a short periodAbnormal period of excessive rainNatural dam where the river is obstructed (ice dam, volcanic flow, landslides)Human effects like clear cut logging
29 Preventing Floods Flooding can be prevented by: Planting trees to help rain absorptionDams to control river flowArtificial levees (eg. sandbags) to raise banksSpillway channels to take the overflow and re-route it