GEOL 101 1nd Edition Lecture 19 Outline of Last Lecture I Radioactivity II Geologic Time Scale Outline of Current Lecture I Hydrologic Cycle II Running Water III Stream Flow Current Lecture Hydrologic Cycle The hydrologic cycle is a summary of the circulation of Earth s water supply The cycle is driven by energy from the sun whereby the atmosphere is the link between the ocean and the continents Processes involved in the hydrologic cycle Precipitation condensed water molecules that fall from the atmosphere to earth Evaporation the transfer of liquid water at the surface to water vapor in the atmosphere Infiltration the movement of surface water into the ground through the soil or rock fractures Runoff water that flows over the surface and does not infiltrate into the subsurface Transpiration the release of water vapor into the atmosphere by plants These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Running Water Begins as sheet flow thin broad sheets of water flowing across the ground Amount of sheet flow depends on the infiltration capacity of the soil how much water can seep into the ground Infiltration capacity is controlled by o Intensity and duration of rainfall o How wet or dry the soil was before the rain o Soil texture o Slope of the land o Nature of the vegetative cover Sheet flow develops into tiny channels called rills which eventually form larger channels Drainage Basins A drainage basin is the land area that contributes water to a stream Drainage pattern consists of the interconnected network of streams in an area Drainage basin of one stream is separated from the drainage basin of another by an imaginary line called a divide o Divides range in scale from ridges separating 2 small gullies to continental divides Stream Flow Streams may flow in one of two ways depending on the velocity Laminar flow slow moving water moves in a relatively straight line Turbulent flow faster moving water is more erratic and may have whirlpools eddies and rapids Stream flow velocity is fastest in the center of the channel Factors that determine velocity Gradient slope or steepness of the stream Channel characteristics including shape size and roughness Discharge volume of water moving past a given point in a certain amount of time o Calculated by finding the cross sectional area and multiplying the area by the stream velocity Changes from upstream to downstream Profile o Cross sectional view of a stream o Viewed from the head headwaters or source to the mouth of a stream o Profile is a smooth curve o Gradient decreases downstream i e gets flatter Changes from upstream to downstream As we go from the headwaters down towards the mouth o Discharge increase as a result of more tributary streams that drain into the main stream o Channel size increases to accommodate more water from the tributary streams o Gradient decreases to a more gentle slope o Channel roughness decreases with a gentler slope Base level is the lowest point to which a stream can erode o Two general types of base level Ultimate sea level Local or temporary lakes resistant rock layers larger main layers Work Done Streams are the earth s most important erosion agents Streams erode transport and redeposit massive amounts of sediment Erosion by streams Stream currents may dislodge clay silt sand and even larger particles from the stream bed and carry those particles along with the current o Stronger currents pickup small and larger particles o Weaker current can only pick up clay and silt Stronger currents carrying larger particles are able to scour and erode the stream bed faster than slower currents carrying small particles Transport of sediment by streams o Material that is being transported by the stream is called the load o Types of load Dissolved load material that is a solution from the chemical erosion of soluble rocks limestone marble rock salt rock gypsum Suspended load Silt and clay sized particles that are carried with the current Bed load coarse particles sand sized increase more along the bottom of channel o Capacity the maximum load a stream can transport Deposition of sediment by a stream As a stream loses velocity such as when it reaches its base level it cannot continue to carry its load Sediment begins to drop out and form deposits known as alluvium Stream sediments are generally well sorted Deposition may occur in any number of areas along the stretch of the river Examples include point bars braided streams sand and gravel bars deltas Areas where alluvium is deposited Channel deposits Bars o May be in the middle of a channel o May be on the inside of a stream meander Braided streams many channels that interlace very commonly associated with glaciers and deltas Deltas fan shaped areas of deposition of mouth of river that flows into the ocean