Sculpting Earth’s Surface Rivers Sculpting Earth’s Surface

Rivers of the World Not a large proportion of Earth’s water A major contributor in sculpting the landscape

Types of Flow Turbulent Flow Laminar Flow Low velocity High velocity Particles flow in straight lines No mixing Rare in water systems Turbulent Flow High velocity Particles move in irregular paths Mixing Most common type of flow Laminar flow occurs when the water particles move along straight lines, called stream lines that are parallel to each other. Turbulent flow tends to occur when the velocity of water and the vicosity and friction are low.

Flows in Rivers Velocity Factors in velocity distance per unit of time (ft/s or m/s) Factors in velocity gradient Velocity. Stream velocity is simply the distance that the water in a river or stream travels over an interval of time, measured in ft/s or m/s. The velocity is related to the discharge of a river, the slope angle of the river bed, and the type of flow. size & roughness discharge Fastest in center; slowest at edges

Gradient Gradient (slope) vertical drop over distance Longitudinal profiles, as shown in below, provide a useful way of looking at rivers. The slope, or gradient, of the river is highest near the headwaters and lowest at the mouth of the river, where it is terminated in a larger body of water, such as a river or ocean. Gradient As discussed previously, streams and rivers tend to flow from a source in a higher elevation, called the headwaters, downward towards a terminus, called a base level, at a lower elevation. The gradient of a stream is a measure of how far a river drops in elevation as it travels a certain horizontal distance. The gradient is the drop in elevation of the river, divided by the run, or horizontal distance the river has traveled, as shown in the equation below: Gradient (m/km) = rise / run = elevation change / horizontal distance The gradient of a river tends to be highest at the headwaters; decreasing to essentially zero at the base level, as shown in the longitudinal profile. Gradient (slope) vertical drop over distance highest @ headwater; lowest near base

Channel Characteristics Size larger channels have more efficient flow Roughness Smooth channels promote more uniform flow Irregular channels create turbulent flow

Discharge Volume of water flowing past a certain point in a given unit of time (m3/s)

Erosion, Transportation and Deposition Rivers typically display three different behaviors Erosion – breakdown and removal of material Transportation – movement of material down a river Deposition – accumulation along banks and beds of rivers

Anatomy of a River Headwater Plains Delta high slope area erosion dominant Plains gradient decreases transportation dominant Rivers tend to start in mountainous or high slope areas. This part of the river is called the headwaters of the river. The river channel tends to be steep and V-shaped, and the bed is rough and irregular. As the river exits the foothills, it flows into the plains. The gradient of the river in the plains tends to decrease rapidly, and the river channel broadens and deepens. The river bed is smoother and the path of the river often meanders. Eventually the river flows into the base level, forming a delta. The delta is a feature that forms when the sediment carried by the river is deposited at the base level (sea level or a lake). Delta Base level Deposition dominant

X-section of a River Features Constantly decreasing gradient Overall, smooth and concave upward curve

Erosion Particles are dislodged from channel and lifted The stronger the flow, the more particles More particles, increase erosion (sandpaper)

Transportation Bed load – moves along bottom by skipping or sliding Streams typically carry large amounts of rock fragments and sediment as they move towards their base levels. This material is called the stream load, and it is composed of bed load, suspended load, and dissolved load. Bed load – moves along bottom by skipping or sliding Suspended load – fine sediment carried above channel – silt and mud Dissolved load – sediment carried in solution - salt

Transportation Criteria of a stream’s ability to carry a load Competence Measures maximum size Related to velocity Capacity Maximum load Related to discharge

Transportation - Landforms Braided Stream Complex stream channels separated by sand bars Coarse material; bedload Meandering Stream Curving stream that migrates side-to-side Fine grained; suspended load Oxbow Lake A curved lake produced when a stream cuts off a meander

Landform Formation Examples of erosion and deposition: Cutbank – banks that are eroded Point Bar – areas of deposition

Fluvial Landforms Examples Meandering Oxbow Braided

Deposition When velocity approaches 0 m/s, deposition occurs Base level – limit to where a stream can no longer erode Sea level – ultimate base level Lakes and dams local base level

Drainage Basins Drainage Basin – The surface area that contains tributaries and supplies the surface runoff that feeds the river Every river system is provided with water from a variety of sources. These include tributary streams, surface runoff, and groundwater. The surface area that contains the tributaries and supplies the surface runoff that feeds the river is called the drainage basin. Typically, ridges that form a valley or bowl-shaped depression that channels the water towards the river bound the drainage basin Dendritic – irregular branching; uniform underlying material Radial – diverge from a central area; volcanic domes Rectangular – right angle bends; contains joints and faults Trellis – parallel; alternating bedrock

Floods New Orleans, 2005 Overflow of a stream channel when discharge exceeds channel capacity The most common and destructive geologic hazard

Floods Causes of Flood Major storms Rapid snow melt Hurricane Sepulveda Basin Causes of Flood Major storms Rapid snow melt Hurricane Pittsburgh 1996 Floods are among the most difficult natural hazards to mitigate although they have occurred since the beginning of recorded history. Property damage in the U.S. from floods topped $12 billion in 1993, and the number of deaths due to flooding is the only natural hazard-related death figure to rise on an annual basis since 1940. Floods can destroy entire towns and cities. In 1931 the flood of the Hwang Ho River, in China resulted in more than 4 million people drowning when it suddenly overflowed its banks in a violent flood. The lethal nature of this flood was due to the unconfined nature of the Hwang Ho River valley. The Hwang Ho flowed over a flat plain as it moved toward its terminus. The level of the channel bed had also risen with time, resulting in a greater sensitivity of the area to flooding. When increased discharge caused water levels to rise, there was little to hold back the flood waters.

Flood Control Artificial Levees Channelization