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Lecture Notes 12 Streams

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Lecture notes ‐ Bill Engstrom: Instructor Streams GLG 101 – Physical Geology Running Water ‐‐‐‐ part of the Hydrologic Cycle Stream ‐ a body of running water that is confined in a channel and moves downhill under the influence of gravity (i.e. water flows downhill when unrestricted at the surface). Streams do not have size in the definition. Rivers are large streams; brooks, or creeks are also streams. So…… Where does the water go after it comes down as rainfall? Water that precipitates on Earth…… then • ‐some infiltrates into ground (groundwater) • ‐initially may "sheet wash" overland • ‐eventually channelizes into rills or rivulets or gullies and then into streams Stream Dynamics – How they work • Headwaters – origin of stream • Mouth – end of stream (e.g. lake, ocean, etc.) • Trunks & Tributaries – stream courses leading into the main stream Basins and Divides • Drainage Basin = area in which all water flows into a particular stream of interest • Drainage Divide = higher areas separating adJacent drainage basins. o Continental Divide – large scale divide (separates the continent – east/west). Flooding ‐ Predominant danger of streams Flooding = the act of a stream breaching it's channel boundaries or increasing beyond its usual depth. Dangers of Upstream Precipitation‐ Floods Consider the many tributaries of the Mississippi. Who’s weather should they be watching in New Orleans (downstream)? We should be watching the weather to see how much is falling and has fallen upstream. This is true wherever you are along a stream. Flooding can occur without warning where you are. It could be completely dry in your area, and a flash flood can occur. People have been caught in floods unexpectedly in many areas, such as in dry “slot canyons” in the arid west (e.g. Arizona). Stream dynamics (Behavior) is primarily controlled by discharge and velocity. Velocity • Stream velocity‐ rate of flow in feet / second. Here are some of the factors that affect velocity. • Velocity is affected by Stream gradient (stream slope)= longitudinal profile = topographic profile parallel to stream flow • Velocity is affected by Channel geometry = Cross profile = topographic profile perpendicular to stream flow • Narrow channels‐ less drag, faster velocities • Wide channels ‐ more drag, decreased velocities • Rough channels choked with debris‐ more drag decreased velocities • Stream Competence = the size of the material a stream is capable of moving . Load = transported material Competence increases with velocity . Larger material gets moved with higher velocities. Can tell what stream is/was capable of moving by looking a particles in channel. Stream velocity affects streams ability to erode/transport/deposit. Stream velocity & discharge regulate stream erosion and deposition • Higher velocities and discharges = erosion • Lower velocities and discharges = deposition Discharge Discharge = cross sectional area of channel (square feet) multiplied by velocity = cubic feet per second. • Affects stream's ability to carry sediment‐ high discharge = higher stream capacity (greater volume of sediment transport). • Dependent on size and climate of drainage basin. Basically, there are two types of streams….. Perennial Streams (continual discharge) Intermittent Stream (irregular discharge) Stream capacity = amount of sediment a stream is capable of moving. This is the amount (NOT the size of the material as with “competence”. • Capacity increases with discharge Low gradient versus High gradient streams High Gradient stream characteristics • Stream competence high (moves larger material). • Stream cuts into channel deepening it. • Cuts down to base level = Downcutting. • Forms V‐shaped channel or narrow V slot canyon depending on rock types. • Increase in discharge results in deeper water Low Gradient stream characteristics and Base Level • In a low gradient stream, the channel is near base level. • Low stream velocity and competence. Base Level = downward limit to stream erosion. We usually talk about sea level as base level, but there are also “temporary” base levels. Ultimate – Ocean / Sea Level Temporary – e.g. lakes or where streams meet tributaries Low gradient versus High gradient streams. In low gradient, lower velocity streams, material is deposited (depositional). In high gradient, higher energy/velocity streams, material is eroded (erosional). Types of Low‐Gradient Streams Meandering Streams = stream capacity sufficient for supply of sediment Characterized by oxbow lakes & cutoff meanders Braided Streams = stream capacity is lower than supply of sediment. Stream braids into multiple channels Most streams do not flow in a straight line. This curvature causes an interesting phenomenon‐ streams will move through time. • Outside bend of stream = fastest velocity, most erosion, deepest channel. This is where the bank erodes. • Inside bend of stream = slowest velocity, deposition, shallow channel . This is where material (sand, silt, clay etc.) is deposited. If you a rafting, tubing or running a river in a boat, you need to be aware of where the deepest and fastest areas are, to avoid bottom hazards. This could be particularly harmful to you if you’re tubing. Remember, the deep water is on the outside bank/bend. Lateral Movement. • Migration of stream meanders ‐ Meanders migrate laterally over time and as they do, they erode and cut a floodplain. • Cutoff meanders and OXbow Lakes – As streams migrate, the meanders often get cutoff. When they do, the water stops flowing through the meander and an oxbow lake is formed. So….What is a flood plain? It is a relatively flat area over which flooding occurs in low gradient streams‐ otherwise, floodplains are dry. Building on a dry floodplain may seem like a good idea, but streams will eventually flood. This is a potential hazard if you are buying or building a home. You should always be aware of the potential flood hazards as a consumer. What happens in a river delta environment? For example, in the Mississippi Delta there are coalescing subdeltas (shifting). The present course wants to shift to path of Atchafalaya River basin. In that case the Mississippi River and New Orleans and Baton Rouge would be bypassed. New Orleans is a maJor port and both cities would suffer from the loss of the river. Dam control structures built have been built to stop nature from taking its course and channel breakthrough. Alluvial Fans that form along steep mountain fronts are another form of stream environment, although water flow is intermittent. Landscape Evolution – A lot of things can change over time that can affect river systems. Changes in Base Level ‐ local or ultimate, cause changes in stream velocity which can change action of stream from depositional to erosional or vice versa • Downcutting‐ effort of a high gradient stream to reach base level. • Headward Erosion‐ erosion parallel to longitudinal profile in direction of headwaters • Lateral Erosion‐ erosion perpendicular to longitudinal profile‐ valley widening in low gradient stream Landforms developed by streams ‐ Streams that apparently cross mountain ranges. Water Gaps can be found which are steep walled notches that cut through ridges ‐‐ How do these form? Here are some different types of streams and methods by which this can happen. • Antecedent Streams ‐ stream precedes development of topography. Stream cuts through mountains as they are built, OR • Superimposed Streams‐ topography buried and exhumed by streams (on pre‐existing structure) • Stream Piracy (aargh) ‐ headward erosion and capture of one stream by another. Stream Terraces are caused by small abrupt changes in base level These are the stages of terrace development…. • Formation of orginal floodplain • Base Level drops erratically (not steadily)‐ stream becomes erosional and downcuts to new base • Lateral Erosion‐ cuts new flood plain leaving old one high & dry Terraces are Oldest surface on top, youngest on bottom. This is the opposite of what we learned about the principle of superposition. Landscape Evolution: Stream Rejuvenation. Changes in base level can also produce incised meanders. Incised Meanders ‐ meandering channels in steep, narrow bedrock valleys. These are common on Colorado Plateau. There are two primary hyposthesis that can eXplain how landscapes form as a result of stream erosion. Cycle of Landscape Evolution‐ Hypothesis 1: Change of slope angle • Initial incision of slot canyon (youthful) • Mass wasting on sides decreasing gradient of slopes‐(mature‐old age) • Streams stay in more or less same place until peneplaination Cycle of Landscape Evolution‐ Hypothesis 2: Parallel Retreat of Slopes • Initial incision of slot canyon (youthful) • Lateral Erosion‐ widens valley maintaining slope angle (mature‐old age) • Streams move throughout history These are some of the factors that influence landscape development • Rock type‐ resistance to erosion • Climate‐ how much water is available and how much chemical weathering is taking place • Stream competance and capacity ‐ size of drainage basin, stream gradient. Drainage Patterns ‐ indicate something about the geologic features underlying the landscape • Dendritic‐ uniformly eroding rock types‐ drainage pattern looks like tree branches • Radial‐ erosion of area surrounding a conical mountain‐ typically a volcano; drainage pattern looks like spokes emanating from a central hub. • Rectangular ‐ stream channels favor faulted or cracked surfaces. Sharp corners in stream patterns • Trellis ‐ main streams flow down valleys in folded rock units 8/2011 .
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