STREAMS AND FLOODS OH MY!!! TYPES OF RUNNING WATER Running water is the most important agent in eroding, transporting, and depositing sediment and developing landscapes. Stream- Any body of running water confined to a channel (of any size), often starting in mountains and ending at the sea. Headwaters- Place where streams start (often in the mountains). -Narrow, V-shaped stream valley -Steep banks made of bedrock. Mouth or delta- Place where the stream enters the sea. -By this time, the stream valley has changed shape into a broad, flat- bottomed stream valley. Sheetwash- Running water that is not confined to a stream. During heavy rains, water may move unconfined over land as sheets of water DRAINAGE BASINS DRAINAGE BASINS Drainage basin (also called a watershed)- The total area of land drained by a stream and all of its branches or tributaries/feeders. Drainage basins are separated by topographic highs called divides. For example. the Continental Divide separates streams that flow to the Pacific from streams that flow to the Atlantic. DRAINAGE PATTERN Drainage Pattern- Pattern formed by a stream and all of its branches and tributaries. Drainage patterns are controlled by the rock or sediment underlying the stream so they tell you about bedrock. 1) Dendritic - Treelike pattern that forms on rock or sediment that easily erodes (very common). 2) Radial- Pattern that mimics the spokes of a wheel. They form around conical features (common in volcanic terrains). 3) Rectangular- Pattern of angular (often 90) bends of tributaries due to fractured bedrock which serves as pathways for streams (common in granitic/basaltic bedrock). ARE STREAMS FEATURES OF EROSION OR DEPOSITION? IT DEPENDS ON SEVERAL FACTORS: 1. VELOCITY OF THE STREAM Velocity is the key factor that determines a streams ability to erode, transport, and deposit sediment. Average speed of stream is ~ 5 km/hr. During flood stage, speed may be up to ~25 km/hr. The maximum velocity of a stream is at the center on straight segments and at the outside curved segments. Each size of sediment (clay, silt, sand, gravel) has certain speed requirements for erosion, transportation, and deposition. GRADIENT 2. GRADIENT Gradient is the slope of a stream (rise/run). It is usually measured in feet/mile. Gradient decreases as stream moves from the mountains towards the sea. Mountain stream gradients are 50 to 200 ft/mile whereas lowland stream gradients are less than 5 feet/mile. Streams with steeper gradients have higher velocities. CHANNEL SHAPE AND ROUGHNESS 3. CHANNEL SHAPE AND ROUGHNESS Flowing water drags against stream channel and is slowed down by friction. The more surface area of the confining channel, the slower the stream moves. Therefore, narrow, deep channels produce fast moving streams and wide, shallow channels produce slow moving streams. Streams will flow rapidly over a smooth channel bottom and more slowly over a channel bottom filled with boulders. STREAM DISCHARGE Discharge is the volume of water that flows past a given point per unit of time. Discharge = average stream width (ft) x average depth (ft) x average velocity (ft/sec) The units of discharge are ft 3 /sec (cfs) or m 3 /sec. Example: A stream that is 100 ft wide, 15 ft deep, and flows at 4 mi/hr (6 ft/sec) has a discharge of 9,000 cfs. STREAM EROSION There are 3 primary ways that streams erode into their channel: 1) Hydraulic action flowing water pries rocks loose. 2) Solution flowing water may gradually dissolve some rock types or cements. 3) Abrasion - flowing water causes the stream channel bedrock to erode from impact with the sediment load (ex. potholes). MECHANISMS FOR TRANSPORTING SEDIMENT A streams sediment load can be classified as: 1) Bedload- Large or heavy sediment particles that travel on the streambed (sand and gravel). Bedload moves by traction (rolling, sliding, dragging) or saltation (bouncing). 2) Suspended load- Fine-grained sediment light enough to stay suspended in turbulent water (silt and clay). This load stays suspended due to the turbulent motion of water making streams appear muddy. 3) Dissolved load- Soluble material which is transported in solution and may end up in the ocean. Vast amounts of sediment are dumped into the ocean each year. It is estimated that rivers in the US carry 250 million tons of solid load (bedload + suspended) and 300 million tons of dissolved load each year. TYPES OF DEPOSITS MADE BY STREAMS 1) Bar deposit: A stream deposit consisting of sand or gravel deposited in the center of the channel. Bars form when the streams velocity or discharge decreases and its bedload is dropped. Braided stream: A stream that has many bar deposits. Braided streams are wide, shallow, and heavily loaded with sediment. Braided streams are characteristic of deserts and mountainous regions where sand and gravel are abundant. TYPES OF DEPOSITS MADE BY STREAMS 2) Meandering Stream: A stream that is sinuous. They develop in areas with low gradients (lower energy than braided streams). MEANDERING STREAMS Red River on the Texas-Oklahoma border. MEANDERING STREAMS Meandering streams carry a lot of silt and clay and look muddy. Water in meandering streams moves faster on the outside of the meander than on the inside of the meander. Erosion occurs on the outside of the curve while deposition occurs on the inside. The sediment in the inside of curves produces point bars. As meanders become extremely sinuous, there is more and more chance that the river will choose a shorter path and cut thru a meander (meander cutoff). The isolated meander then becomes a crescent-shaped oxbow lake. FLOODPLAIN DEPOSITS 3. FLOODPLAIN DEPOSITS Floodplain: The area on both sides of a stream channel. Typically this area is dry, but in times of flood the stream will overflow its bank and the floodplain will be covered with water. As the water recedes, it leaves behind silt and clay. Thus, floodplain deposits build successive, horizontal layers of silt and clay. The coarsest sediment is deposited directly adjacent to the stream and may form a natural levee. . DELTA DEPOSITS 4. DELTA DEPOSITS Delta- A deposit where a stream flows into a large body of water and is forced to drop its load. Deltas often have a triangular shape in map view, like the Greek letter delta, D. Deltas are sand-rich and can be braided, just like braided streams. ALLUVIAL FAN DEPOSITS 5. ALLUVIAL FAN DEPOSITS Alluvial fan- A large, fan-shaped pile of sediment that forms where a stream emerges from a narrow mountain canyon. The stream loses it energy as it emerges from canyon and drops its gravelly sediment load. As the fan builds outward, the stream will move out over the fan, depositing more sediment. Braided stream channels cover the top of the fan Deposition on the fan is discontinuous due to infrequent rainstorms. Alluvial fans are best preserved in the deserts of the southwestern U.S. FLOODING Flooding is beneficial in the sense that it replenishes nutrients on the floodplain and provides fertile planting ground. Flooding can also be detrimental to towns built along the streams. Urbanization contributes to flooding by paving land with concrete. The concrete reduces infiltration and increases runoff. DEVELOPMENT OF STREAM VALLEYS All streams travel on a gradient toward the lowest elevation. Streams attempt to flatten out the gradient by downcutting, or eroding into the stream bed, creating a valley. The lowest level to which a stream may erode is called base level. Base level for streams that travel to the ocean is sea level. However, base level does not have to be sea level in cases where streams flow to a lake or dry valley (for example, Death Valley). DEVELOPMENT OF STREAM VALLEYS Streams cannot erode below base level or else they would have to flow upstream to reach their final destination. Streams with large gradients (mountain streams) downcut more than streams with gentle gradients (meandering streams). Streams will begin rapid downcutting and erosion when base level drops due to either a drop in sea level or uplift of the land. Meandering streams deposit huge amounts of sediments on the floodplain. However, with a drop in base level, a meandering stream will begin downcutting again, producing incised meanders. DEVELOPMENT OF STREAM VALLEYS Lateral erosion is a second way in which a stream valley develops. The stream will undercut the banks, causing the overlying rocks to fall down (mass wasting). Over time the V-shaped valley becomes wider and the floodplain grows. The river can meander back and forth within this valley.