Streams Nancy A. Van Wagoner Acadia University

Distribution of Earth's water n What are the percentages? -Oceans - Glaciers - Groundwater - Lakes and Rivers - Atmosphere 97.5% 1.8% 0.63% 0.02% 0.001%

Importance of fresh water resources Life on earth Carve landscape Transportation Recreation Electricity FISHERY Irrigation-agriculture

Hydrologic Cycle (Fig. 11.1) n Continuous movement of water between the oceans, the atmosphere and land n The system is sun-powered. n Values in 1000's of km 3 /yr.

Streams and their characteristics n stream = an channelized flow, regardless of size n river = large stream fed by smaller tributaries flood plain banks bed

Characteristics Continued n wet year round versus intermittent

Work of Streams n erode material n transport material n deposit material

Stream Velocity n Important because it is directly related to the ability of a stream to do the work of streams.

Velocity Distribution n see figure n the velocity distribution is controlled by the frictional drag of the -air -bed -banks

Top view Maximum velocity at the centre Side view Maximum velocity is just below the surface

Factors Controlling Velocity: n slope or stream gradient n channel size and shape n roughness of the bed and banks n discharge

Factors Controlling Velocity n Stream gradient = slope of stream -drop/unit distance m/km -slope is directly proportional to velocity streams cutting newly uplifted areas: high gradient/velocity Mississippi - lower gradients < 1 m/km Meters drop Kilometer of horizontal distance

Factors Controlling Velocity n Channel size and shape -determines amount of water in contact with the bed and banks -The most efficient channel: least perimeter for cross sectional area -The most efficient channel space: large semicircular shape -DIAGRAMS: 3 streams with same cross sectional area, different perimeter

h w 1 unit 10 units 2 units 5 units 2.5 Cross-sectional area Perimeter h x w R2/ h + w + h R

Factors Controlling Velocity n Roughness of the Channel -refers to the size of particles lining the channel -increasing roughness, increases frictional drag

Factors Controlling Velocity n Discharge - volume of water flowing past a given point/unit time -Q = cross sectional area x velocity = m 3 /sec Q = A x V -Flood: more water: size of channel increases: velocity increases therefore Q increases -Drought: deposition size of channel decreases therefore frictional drag increases, and velocity decreases therefore Q also decreases

Changes downstream: (fig. 9.5) n streams adjust to maintain a balance between all factors that govern their flow n Near Headwaters: least discharge steepest gradient narrow, shallow channel n Downstream: number of tributaries increases therefore discharge increases –recall Q = A x V – V would increase drastically if A and gradient were not adjusted, therefore stream widens and deepens, increasing A gradient decreases

Sediment size related to: n stream energy n transport distance

Stream Erosion - Three Processes see fig n 1. Hydraulic Action: erode by lifting unconsolidated material due to impact of water n 2. Abrasion: sand paper effect due to sediment in water n 3. Solution: minerals dissolved in water

Sediment Transportation (fig. 11.5) n 1. Dissolved load -carried in solution -acquired mostly from groundwater -~ 20 % of total load n 2. Suspended load -most of load -fine-grained: silt and clay n 3. Bed load -rolling and sliding -saltation

Ability of a stream to erode and transport material is established by: n 1. competence: maximum size -competence is proportional to velocity n 2. capacity: maximum load -capacity is proportional to discharge

Base Level (Impt. Concept) n ultimate point to which a stream can erode its channel n Ultimate base level = sea level n temporary base level examples: -lake, reservoir, internal drainage basin n Any change in base level causes the stream to adjust its channel. n See diagram (fig )

Deposition and Depositional features n deposition occurs when velocity decreases n competence decreases n and some suspended sediments settle out

Alluvium n unconsolidated sediment deposited by a stream

Depositional Features n Delta: form where stream enters ocean or lake; velocity suddenly reduced; result = deposition of a triangular wedge of sediment called a delta (fig and 11-11) -Important! The shape and extent of a delta is a balance between: A. deposition rate of deltaic sediment, and B. rate at which sediment is removed by erosion -Large rivers tend to have large deltas

Depositional Features n Alluvial Fan -Fan-shaped alluvial deposit at the base of a mountain or hill -Forms where mountain stream reaches the plain -Gradient abruptly lowered -rapid deposition occurs n Very unstable for building development due to: -poorly consolidated materials -shifting of distributary channels

Depositional Features n Natural levees and flood plains -Periodically, rivers overflow their banks and deposit sediment -Natural levees can be 6 m+ high. -The area behind the levee is poorly drained = back swamp.

Stream Valleys: Two General Types n 1. narrow v-shaped n 2. wide valleys with flat floors -there is a gradation between the two types

Type of valley gives indication of the amount and type of work the stream is doing: n v-shaped: down cutting n wide/flat: lateral erosion

narrow valleys: characteristics n rapids n waterfalls n steep gradient; active faulting/uplift, rock resistance n above sea level

wide valleys: characteristics n Form when stream channel is cut close to base level; n down cutting - less dominant; energy is directed side to side n Result = widening valley, formation of a flood plain n River confined to channel except during flood.

Meandering Streams n Type of stream that flows on broad flood plains. n Meanders always changing position because the maximum velocity is directed outward toward the bank. n Result = meanders move sideways and slightly downstream n If one meander moves downstream slightly faster than another then: -Cut off meander and oxbow lake may form -Eventually leaves a meander scar

Braided streams n Most streams are braided to some extent. n Very common in arid regions; -lots of unconsolidated sediment; -little water -low frequency/high volume rainfall

Drainage patterns n give an indication about geology -dendritic -trellis -radial