Streams Why study streams? Running water is the most important geologic agent in erosion, transportation and deposition of sediments. Water The unique physical and chemical properties of water make it the single most important chemical compound in defining our existence on this planet. These properties include: Water exists in all three states (vapor, liquid and solid) at Earth s surface. Water is a polar molecule that makes it a nearly universal solvent. Unlike most compounds, water expands its volume upon freezing. Hydrologic Cycle Represents the circulation of water in its three physical states (vapor, liquid and solid) through the Earth s atmosphere, hydrosphere and lithosphere. See figure 10.1 1
Driving Force for Hydrologic Cycle The hydrologic cycle is driven by radiant energy from the Sun and Earth s gravity. Radiant energy of the sun is converted into stored energy of water vapor through evaporation. Stored energy of water vapor is converted to kinetic energy through condensation and precipitation. Distribution of Water Distribution of water in the hydrosphere: ~97% Oceans ~2% Glaciers ~0.6% Groundwater ~0.02% Streams and lakes Terminology of Streams Stream - body of surface water flowing in a confined channel (independent of size). Stream channel - long narrow depression eroded by the stream into rock and sediment. Stream banks are the sides of the channel. Stream bed is the bottom of the channel. 2
Terminology of Streams Flood plain is the stream valley inundated during floods. Tributary - stream that flows into a larger stream. Sheetwash - thin layer of unchanneled water flowing downslope. Stream Profile Longitudinal profile (fig. 10.2A) Headwaters - upper part of stream near its source. Mouth - place where stream enters ocean, lake or larger stream. Cross-sectional profile (fig. 10.2B&C) Near headwaters - steep mountain, V-shaped valley cut into solid rock. Near mouth - broad, flat-floored valley filled with sediment. Channel Patterns Straight channels Rare, most often streams with steep gradients in resistant rock. Meandering channels Frequently found in streams with gentle gradients in easily eroded sediments. Braided channels Flow along many shallow, interconnected channels where more sediment is supplied to the stream than it can carry. 3
Drainage Drainage basin - total area drained by a stream and its tributaries Drainage Pattern - can reveal information about underlying rock type and/or structure. See fig. 10.4 Common Drainage Patterns Dendritic Tree branches-like drainage pattern. Common in uniformly eroded regions. Rectangular Tributaries have frequent right angle bends. Common in regularly fractured rocks. Trellis Tributaries perpendicular to parallel main streams. Common in folded strata of variable resistance. Radial Streams diverge outward like spokes of a wheel. Common around composite volcanoes and domes. See fig 10.5 Stream Erosion, Transport and Deposition Kinetic energy of water flowing downslope performs work in the form of eroding regolith and transporting sediments. Governed by stream velocity and particle size. Velocity - the rate at which water flows. See fig. 10.7 4
Stream Velocity Three factors control stream velocity: Stream gradient Steepness of the stream (vertical drop over a horizontal distance). Discharge Volume of water flowing past given point in unit time. Discharge = velocity x cross-sectional area of stream Shape and roughness of channel Friction between flowing water and stream channel slows velocity. Velocity in a Stream Channel In straight sections Velocity is greatest near middle of channel (least friction with channel). In curved sections Velocity is greatest on outside of curves. Cutbank -steep slope caused by erosion along outer bank of curve due to greater stream velocity. Point bar - series of arcuate ridges of sand or gravel deposited on inside of curves due to lower velocity. See fig. 10.6 Stream Erosion Mechanisms Hydraulic action Ability of flowing water to lift and move rock and sediment. Solution Chemical dissolution minerals/rocks. Abrasion Wearing away of channel and grinding of sediments by friction and impact of load. 5
Stream Load Load - all material transported by a stream. Capacity - quantity of sediment that stream carries in a given time. Competence - largest-sized particles a stream can carry at a given time. Stream Transport Transport of sediment load in a stream occurs by (fig. 10.14): Bed load - large or heavy particles that travel along the bed. Traction - moving by rolling, sliding or dragging. Saltation - sand-sized particles that move by series of short leaps or bounce along stream bed. Suspended load - finer (clay-silt) sized particles carried by turbulence of flowing water. Dissolved load - dissolved ions carried in solution. Stream Deposition Deposition of sediments depends on the type of load: Deposition of clastic materials Decrease in stream velocity as it approaches mouth decreases ability to transport larger particles. Results in well-sorted deposits. Deposition of dissolved load Dissolved ions are deposited by evaporation or precipitation by chemical reactions. 6
Depositional Features Three types of stream deposits: Channel deposits Form in stream channel itself. Flood plain deposits Accumulation of sediments on a flood plain adjacent to the stream channel. Alluvial fans and delta Form where stream s gradient rapidly decreases like when it empties into a lake or flat plain. Channel Deposits Bar - elongated bar of sediments, generally a transient feature. Point bar - deposit on inside of curve in stream. Mid-channel bar - sand/gravel deposit in middle of stream channel. Braided stream - flow along many shallow, interconnected channels instead of single central channel. Form when sediment supply is greater than stream can carry (fig. 10.18). Meandering Streams Meanders - pronounced sinuous curves in stream channel (fig. 10.22). Form by erosion on outside and deposition on inside of stream curves. Oxbow lakes - form when stream meanders become cutoff during flooding (fig. 10.24). 7
Flood Plain Deposits Flood plain - broad strip of land built up by sedimentation on either side of a stream channel. Natural levees - low ridges of flooddeposited sediment that form on either side of stream channel See fig. 10.28 Alluvial fan and Delta Deposits Delta - triangular-shaped pile of sediment that forms where stream enters a calm body of water (lake, ocean; fig. 10.30). Distributaries - form when stream feeding a delta or fan splits into many channels. Alluvial fan - triangular-shaped pile of sediments that forms where a stream enters a flat plain or valley floor (fig. 10.32). Stream Valley Development Downcutting Downward erosion of stream bed. Base level Deepest level to which a stream can erode its bed. Ultimate base level for most streams is sea level. Temporary or local base levels occur where streams empty into lakes or particularly resistant rock layers. See fig. 10.44 8
Stream Erosion Features Stream Piracy Headward erosion results in a stream capturing the drainage of a nearby stream (fig. 10.48). Stream Terraces Step-like landforms found above flood plain formed as stream erodes its flood plain (fig. 10.50). Incised Meanders Meanders that maintain sinuous curves as stream downcuts. Typically form during uplift (fig. 10.53). 9