Running Water

Hydrologic Cycle The hydrologic cycle is a summary of the circulation of Earth’s water supply ► Processes involved in the hydrologic cycle  Precipitation  Evaporation  Infiltration  Runoff  Transpiration

Running Water ► Begins as sheetflow  Infiltration capacity is controlled by ► Intensity and duration of rainfall ► Prior wetted condition of the soil ► Soil texture ► Slope of the land ► Nature of the vegetative cover ► As Sheetflow continues currents in water develop into tiny channels called rills

Streamflow ► Two types of flow in stream water  Laminar flow  Water flows in straight paths with very little mixing  Turbulent flow Type of flow determined primarily by velocity

Factors that determine velocity ► Gradient, or slope Slope is a measure of how height changes over distance ► Channel characteristics including shape, size, and roughness Discharge – the volume of water moving past a given point in a certain amount of time

Stream Profiles ► Profile is a smooth curve that starts at the head and finishes at the mouth of the stream Gradient decreases downstream

How can velocity increase downstream ► Discharge ► Channel size

Erosion in Streams ► Base level - the lowest point to which a stream can erode ► Two general types of base level  Ultimate (sea level)  Local or temporary ► *Changing conditions causes readjustment of stream activities  Raising base level causes deposition  Lowering base level causes erosion

Base Level

Stream erosion ► Two main processes:  Abrasion  Dissolution ► Stronger currents lift particles more effectively ► Transported material is called the stream’s load

Types of load ► Dissolved load - about 4 billion metric tons of dissolved mineral matter are supplied to oceans each year by stream ► Suspended load  Mostly fine sands, silt and clay-sized particles  Controlled by the velocity of the stream and the settling velocity of the material ► Settling Velocity - the speed at which a material falls through a still fluid  Determined by size, shape and specific gravity

Types of Load ► Bed load- The portion of material that is too large to be carried in suspension.  Material may move by: ► Rolling ► Sliding ► Saltation – jumping or skipping along the bottom

Terminology ► Capacity – the maximum load a stream can transport ► Competence – A measure of the maximum particle size a stream can transport  Determined by the stream’s velocity  In general a streams competence increases by the square of its velocity ► (as the velocity doubles the competence increases by four)

Deposition of sediment by a stream ► Caused by a decrease in velocity  Competence is reduced  Sediment begins to drop out ► Stream sediments  Generally well sorted  known as alluvium

Channel deposits ► Bars – Temporary deposits of sand and gravel  Point Bars- Bars deposited on the inside point of a turn in a stream.

Oxbow Lakes

Channel deposits ► Braided streams – Caused by deposition on the stream bed that eventually ‘chokes’ the channel Caused when:  More turbulent tributary enters a main stream and deposits its bed load.  Debris from barren slope is flushed into stream

Braided Stream

Floodplain deposits ► Natural levees – form parallel to the stream channel by successive floods over many years ► Back swamps ► Yazoo tributaries

Stream Development