Drainage Basins and Streams

Routes for water to enter stream:

• Surface runoff
• occurs when precip rate > infiltration rate, especially in semi-arid to arid areas w/ little vegetation and where soil is shallow or bedrock exposed
• also occurs where surface becomes saturated in prolonged storms or where water table is at surface (usu. near stream, in humid areas)
• surface runoff = fast route to stream for stormflow
• also where pavement, roofs etc. prevent infiltration
• vegetation:
• catches some precip, evaporation back to atmosphere
• transpires water drawn from soil back to atmosphere
• slows down surface runoff and protects surface from raindrop impact and erosion by wash
• Throughflow
• through soil/debris layer above bedrock
• slower route to stream for stormflow
• important where soil is deeper and well-vegetated in humid areas
• Groundwater
• flow in saturated zone below water table to stream
• very slow to extremely slow, constant
• contributes the baseflow part of stream discharge

Discharge = volume of water flowing past a point per unit time

• Discharge = Q = width x average depth x average Velocity = Channel X-section Area x average Velocity = ft x ft x ft/s = ft³/s = cubic feet per second ( = “cfs” in common jargon)
• average discharge in Amazon River = 7.5 million ft³/s
• late Feb 2001 discharge in Rio Grande 800 ft³/s
• measured by stream gages for some streams by USGS (see their websites for discharge of New Mexico and U.S. rivers)

Drainage basins

• divide = boundary of basin
• watershed = basin in U.S., but watershed = divide in U.K.
• stream ordering
• unbranched stream = 1st order
• 2 streams of same order come together to make next higher order, e.g.:
• 1st order + 1st order = 2nd order,
• If more 1st order streams enter the 2nd order it remains 2nd order
• If another 2nd order joins, then 2nd + 2nd = 3rd order - etc.

Drainage density in a basin = total length of all channels/basin area

• greater density indicates high runoff per basin area along with erodible geologic materials
• can have very high density in arid area if erodible materials (lack of vegetation also allows erosion)
• can have very low density in humid area if resistant rock type and/or well-vegetated slopes

Downstream changes in streams

• Discharge increases
• Gradient decreases
• Average velocity often increases slightly (not always), because:
• Channel roughness (thus resistance to flow) decreases
• Sediment size decreases

Sediment load of streams

• Bedload - moves by rolling, sliding, bouncing along bed; coarser sediment (sand-gravel)
• Suspended load - held above bed by turbulence in flow; finer sediment (sand, silt, clay)
• Dissolved load - may be a substantial part of solids carried by stream, especially in areas of readily dissolved rock like limestone

Rivers and landforms

Alluvial streams: channels formed in the stream's own deposits, adjust ~rapidly to changing discharge, sediment supply

Bedrock stream: channel formed mainly of bedrock, changes slowly

Alluvial channel patterns:

· Meandering channels

o form dominated by suspended load transport

o generally moderate to low slope

o relatively deep, narrow channel

o point bars inside bend - form by lateral accretion

o cutbank outside bend

o floodplain surface covered with fine sediment deposited by overbank flow in floods - vertical accretion

o meandering requires "excess" energy

§ meandering reduces channel slope

§ a straight channel in same valley would have higher slope

o meanders abandoned by cutoff across necks - old channel becomes an oxbow lake

· Braided channels

o form dominated by bedload transport

o generally moderate to high slope

o relatively broad shallow channel

o unstable, rapidly shifting bars to moderately stable vegetated islands separating channels - vegetation usually flood-adapted riparian species

o high bedload sediment supply from glaciers, mass movements, volcanism, etc.

o may form where channel destabilized by flooding and/or devegetation

Stream terraces - floodplains abandoned by downcutting of channel

Tread = old floodplain surface; riser = scarp formed by downcutting

• Fill terraces - channel level rises by aggradation (buildup) of sediment in valley; later downcutting leaves fill surface
• Erosional (strath) terraces - lateral migration of stream planes off underlying rock or sediment, leaves thin cover of stream sediment after downcutting

Floods

Overbank flow is common on natural rivers - minor flooding will occur every 2-3 yr on average on many rivers

"100-year flood" = discharge that is equaled or exceeded ON AVERAGE once every 100 years

• can occur in two successive years, or not at all for well over 100 years, as long as average is 1 per 100 years
• there is a 26% chance (or about 1 in 4 odds) that a house on the 100-year floodplain will be flooded during a 30-year mortgage

National Flood Insurance Program attempts to limit development in 100-year floodplain, but several loopholes exist