Drainage Density

1
Drainage Density

• Low (x km/km2)
• Moderate
• Why?
• Climate
• Substrate
• Slope

2
Badlands

• Very high drainage density!
• Why?

3
Watershed Networks

• Watershed network comprised of
• headwater and network systems
• First and second order streams often comprise 70
  of the stream network (Benda et al, 1992)
• High ecological value
• Stream networks defined by
• Natl Hydrography Dataset (1100,00)
• Terrain analysis
• (area, area-slope area-length thresholds)

4
Effects of low order channels on downstream
reaches in the network

• Synchronous (or asynchronous) inflows of water,
  sediment, nutrients, and organic matter create a
  variety of channel conditions and biological
  assemblages
• Connectivity of headwater systems to downstream
  reaches affects the cumulative and dispersed
  nature of material transport processes
• Gomi, et al, Understanding processes and
  downstream linkages of headwater systems,
  BioScience, Oct. 2002, vol. 52, no. 10

5
Abundance related to network position
6
Networks

• A stream or river network is composed and
  characterized by
• A set of nodes classified into (a) outlet node,
  (b) exterior node, (c) interior node.
• Exterior nodes occur in headwater basins.
• Interior nodes include all inner basins with
  multiple tributaries. The outlet node is the root
  of the network.
• A set of links or reaches classified into (a)
  interior links or (b) exterior links.
• A numbering system to order the stream links
  and nodes
• Horton (1945) developed a numbering system
• Strahler (1957) improved upon Hortons
  classification system
• http//www.ees.nmt.edu/vivoni/mst/lectures/Lecture
  18.pdf

7
Network Ordering

• Channels originating at a source are first order
  (headwater) streams
• For Horton-Strahler,
• When stream of same order join, add 1 to the
  stream order
• When stream of different order confluence, assign
  highest order
• For Shreve,
• When streams of similar order join, add them to
  get d/s order

http//www.ees.nmt.edu/vivoni/mst/lectures/Lecture
18.pdf
8
Channel Initiation

• What are the processes whereby water movement
  becomes sufficiently concentrated to cut a
  definable channel?
• Linked to modes of hillslope water movement and
  erosion
• Under what conditions is that initial cut
  maintained and enlarged to initially ensure a
  permanent channel?
• Linked to network development

9
Channel Initiation

• Channel head the upstream limit of concentrated
  water flow between banks (Dietrich and Dunne,
  1993)
• a major boundary between hillslopes and channels
• pivot point in sediment transport between
  diffusive process and incisive process
• transition to incisive processes from diffusive
  processes
• an erosional threshold is exceeded enabling
  channel-forming processes to become effective
• Channel initiation requires runoff
• Channel initiation occurs by
• Horton and saturation overland flow
• Subsurface flow
• seepage erosion
• shallow landsliding

10
Channel Head Location and Topography
Montgomery and Dietrich, 1989
11
Channel Initiation and Basin Morphometry

• Process model for channel initiation by shallow
  landsliding
• convergent topography causes colluvium eroded
  from adjacent hillslopes to accumulate
• at critical threshold, landsliding occurs
  exposing underlying material
• erosion of underlying material by saturation
  overland flow initiates channel
• Channel heads controlled by hillslope process
  rather than network extension
• Inverse of source basin length drainage density