Running Water Hydrologic Cycle The hydrologic cycle is a summary of the circulation of Earths 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 streams 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 streams 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