4 Streams Streams are the major geological agents operating on the surface of the land. The chieffocus of streams are-How water flows in currents-How currents carry sediment-How streams break up and erode solid rock-How streams carve valleys and assume a variety of forms as they channel water downstream
5 Stream Water Flow 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 coverSheetflow develops into tiny channelscalled rills
6 Two types of flow determined primarily by velocity Turbulent flow Laminar flowTurbulent flowFactors that determine velocity– Gradient, or slope– Channel characteristics including shape, size,and roughness
7 StreamflowLaminar – water particles flow in straight paths giving flat, gentle surfaceTurbulent – water moves in an erratic fashion characterized by swirling eddies and whirlpools (white water)
9 Streamflow Factors that determine velocity – Discharge – the volume of water moving pasta given point in a certain amount of time- Changes from upstream to downstream- Profile* Cross-sectional view of a stream*Viewed from the head (headwaters or source)to the mouth of a stream
10 Changes from upstream to downstream -Factors that decrease downstream• Gradient• Channel roughness-Base level and graded streams(Base level is the lowest point to which a stream can erode
11 Two general types of base level – Ultimate (sea level)– Local or temporaryChanging conditions causes readjustmentof stream activities– Raising base level causes deposition– Lowering base level causes erosion
15 Streamflow DischargeDischarge is the volume of water that flows past a given point in a given time.The volume is calculated by multiplying the area water in the stream channel (depth x width) by the velocity (distance/time) of the stream flow.The dimensions of the stream channel are expressed in feet or meters and velocity is measured in feet or meters per second. Consequently the units of discharge are cubic meters (or feet) per second, i.e. discharge is the volume (cubic meters or cubic feet) of water that passes a given point in one second.
16 Discharge (m3/sec) = channel width (m) x channel depth (m) x velocity (m/sec)
17 Discharge can be calculated by multiplying the area of the stream channel in cross section (width x depth) by the distance traveled in a given time (second).
18 Stream Transport Stream “load” Dissolved load – dissolved and carried by the waterSuspended load – suspended in and moved by the waterBed load – moved along the bottom (bed)Flotation load – light material floating on surface
30 Streamflow SummaryDischarge is the volume of water that flows past a given point in a given time.The USGS has over 7,000 stream gaging stations. Data from over half of which are available on-lineStream gages measure the depth of water in a stream channel and the velocity of flowThe recurrence interval is the average time in years between floods of the same sizeA hydrograph illustrates discharge over time
31 Stream velocity may be measured quite simply Stream velocity may be measured quite simply. The technique is to drop a floating object in the stream, time its travel along a meter stick several times, and average the readings
32 Stream velocity varies within a stream channel Stream velocity varies within a stream channel. Maximum velocity occurs on the outside of channel bends; minimum velocity occurs on the inside of bends.
33 Erosion occurs on the outer banks of streams (cut-banks) where velocity is greatest and deposition occurs on inner banks (forming point bars) where velocity is least. The stream channel slowly migrates across the valley floor in the direction of the erosion to form long, looping bends termed meanders.
38 Factors of stream velocity that control erosion GradientChannel shape, size roughnessdischarge
39 Summary of Stream Velocity Stream velocity varies around curves in stream channelsVelocity is greatest on the outside of curves, least on the insideErosion occurs in areas of higher velocity forming cut-banks on the outside of stream curvesDeposition in regions of low stream velocity forms point bars on the inside of curvesErosion and deposition cause stream channels to migrate laterallyVelocity decreases as the length of the wetted perimeter (channel banks and bed) increasesA floodplain is a broad, flat plain adjacent to a stream channel