November 15 lecture

**River Ecosystems-

Theme- Physical processes and ecological processes are intertwined**

Physical setting

  • Three dimensional— upstream to downstream, lateral transfer (movement from riparian to streamside zone), and vertical dimension (water flow from surface to groundwater or vise versa.)
  • Streams can be classified into orders (first order are smalles perennial units, confluence of 1st order streams yields a 2nd order and so on.
  • Vast majority of streams are in lower order (1st, 2nd, 3rd.)
  • Geometry of stream classification is shorthand for talking about river size and drainage (instead of saying, "it's a big river," it's a 10th order… rather than it's a small river, "it's a 1st order river.")
  • River systems are hierarchical- stream "reach" is smaller than a segment, "reach" has series of repeatable units nested within (i.e. riff and pool), and within "reach" you have different habitat (fast flowing riffle has gravel area, sandy area, etc.) What you find in larger unit has influence over smaller unit.
  • Watersheds are classified using HUC catalog (hydrologic unit cataloging)- catalogs all river systems in US. Classifies by water resource region, sub-region, stream reaches and gages.

Fluvial Geomorphology

  • Channel dynamics and channel shape: braided rivers are rich in sediments and river has to "braid" it's way through sediments (water movement appears braided.)
  • Rivers do have regularity with regards to position in longitudinal profile. River system looks different based on sediment/water supply balance. Channel types in longitudinal profile identify interaction b/t slope, discharge, and sediments. Power of river is a function of slope and amount of water going down channel (slope * discharge) and this is what moves sediments. Point where supply and sediments are in balance is where river equalizes.
  • Lane's Law- sediment discharge (total amount of sediment being moved— inorganic material like sand and stones) * sediment average particle size = stream discharge (amount of water in cu m/second) * stream slope (rise/run of riverbank)
  • When these are in equilibrium, channel size stays same. When not in equilibrium, something will happen to channel.
  • Channel equilibrium- suppose climate wetter (more water in system) and slope same, stream has gained power due to climate change, so sediment power has to balance (has to cary more sediments) will scour stream bed. Slope decreases don't change water, lost power, can't carry sediments, and sediment "dumps out."

** Practical examples:**
1. Channel is straightened, Qs or D50 must increase, and stream will degrade.
2. Transfer water into basin, Qs or D50 must increase, stream will degrade.
3. Water export decreases, Qs or D50 must decrease, stream will aggrade.
4. Put in a dam- sediment hungry river, left sediment in reservoir, out comes water from dam carrying lower sediment load.
5. Restoration "re-meanders" a stream, decrease slope, expect sediment deposition therefore. Loss of power.
Know relationship between slope, flow, sediment. Consider how human intervention often changes sediment, slope, or water supply, consequences for channel dimensions, which is why stream restoration projects end up not being very successful.

Biological setting


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