Fluvial Systems and Landforms
• Overland Flow & Drainage Basins
• Hydraulic Geometry & Channel Flow
• Fluvial Processes & Landforms
• Human Interactions with Streams
Overland Flow
• Perennial Streams – water runs all year
• Ephemeral Streams – water runs only part of year
• Sources of stream water:
– Groundwater
– Melting Ice
– Surface Runoff
Drainage Basins• Drainage Basin – area contributing
groundwater and runoff to a stream
• Drainage basins are divided from each other by topographic barriers called watersheds
• Drainage Divide – elevated terrain forming rim around a drainage basin
• Drainage basins vary tremendously in size
• Basins are nested, smaller within larger
Stream Ordering• Represents stream size• Smallest streams in basin order 1 – order rises when 2
streams of same order come together at confluence
Hydraulic Geometry
• Geometric attributes of river channels
• Variables:– w = channel width– d = channel depth– v = velocity of water– s = slope (steepness) also called gradient– Q = discharge (amt of water flowing)– Q = w x d x v (units m3/s or ft3/s)
Stream Hydrograph
• Graph showing fluctuation in stream discharge over time• Lag between storm event and highest discharge
• Base flow – flow rate sustained by groundwater influx
Flooding
• Flood Stage – stream discharge increases so that water spills out of channel onto adjoining ground
• Return Period – time between events of a given magnitude, e.g. annual flood, 50-year flood, 100-year flood
• Larger floods occur less frequently
Mississippi River Floods - 1993
• At least 100-year flood, perhaps a 500-year flood
• Heavy winter rains saturated ground
• Stationary high pressure in Southeast in summer, blocking mid-latitude jet stream over Midwest
• Cool, dry air collided with warm, moist air along jet stream, creating constant precipitation
• Precipitation ran off into stream channels & rivers
Fluvial Processes and Landforms
• Running water is most important geomorphic (landform shaping) process on Earth’s surface
• All landforms due to either erosion or deposition
• Erosional landforms occur when sediment, soil, or rock is stripped away from land
• Depositional landforms occur where sediment accumulates after being dropped
Hillslopes• Most active zones of fluvial erosion due to high
relief creating fast-moving, powerful waterRills Gully
Glacial Landforms
• Rock & debris picked up by glaciers, transported in direction of movement & deposited
• Glacial erosion:– Glacial Abrasion – scratch and gouge bedrock– Glacial Striations – caused by glacial abrasion– Glacial Grooves – deep striations– Glacial Plucking – boulders ripped from ground
by glacier – deposited by retreating glacier, called Glacial Erratics
Alpine Erosional Landforms
• Glacial Erosion:– Cirque – bowl-like feature on mountain flanks– Tarn – small lake in bottom of cirque– Arête – narrow, steep ridges between cirques– Horn – mountain with 3 or more arêtes at summit– Glacial Trough – u-shape valley eroded by glacier
Glacial Depositional Landforms – Glacial Drift
• Glacial Till – sediment directly deposited by glacier – many particle sizes
• Moraine – winding ridge formed by till at the front or side of glacier – Moraine types:– Lateral – along former edges of glacier– Terminal – along front of former glacier– Recessional – formed as glacier recedes– Medial – between 2 glaciers– Ground – irregular deposition as glacier recedes
Glacial Depositional Landforms (outwash)
• Glacial Outwash – sediments deposited by water out & under a glacier as it melts – forms Outwash Plain, flat feature in front of former glacier
• Kame – large mound deposited near glacier front
• Esker – winding ridge from water flowing in tunnel through ice under glacier
• Kettle Lake – big ice block fallen off glacier front is buried by outwash, melts later forming lake