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Hydrologic Processes

Changes in the vegetative cover that influences evapo-transpiration, interception, snow depositional patterns, etc. can change the magnitude, duration and timing of runoff. Urban watershed development can result in a high percentage of impervious surface, that changes the probability, frequency, magnitude and duration of runoff events. The effects of these flow increases can have varying results depending on the morphology (type) of the stream channels and their stability. For example, if streams are incised (G and F) below urban development, then increased flood flows are contained within the channel, causing excess bed and bank erosion, land loss and increased sedimentation. Various stream type delineations are shown in Figures 68 through 70; for review see also Figure 14 (PDF, 192 kb, 1 p.) and Figure 15 (PDF, 168 kb, 1 p.), and Table 3 (PDF, 66 kb, 1 p.) (Rosgen 1994, 1996).

Vegetation and urban changes should be evaluated within second to third order basins to determine potential flow changes. Road density associated with slope steepness needs to be observed by sub-watersheds at this level. This assessment is needed to determine potential change in drainage density due to sub-surface flow interception and change in evapo-transpiration without hydrologic recovery. The potential consequences associated with the processes influenced by an assortment of land uses including streamflow change are shown in Table 9.

Table 9. Relation Between Land Uses/Activities, Processes Influenced, and Consequences

Activity Processes Influenced Potential Consequence
Streamflow decrease in magnitude, duration and timing. Due to reservoirs, diversions Shear stress Down arrow
Stream powerDown arrow
CompetencyDown arrow
Sediment transport capacityDown arrow
  • Excess sediment deposition
  • Accelerated bank erosion
  • Widening channel
  • Successional state
  • Aggradation
Urban - Stream flow discharge increase due to high % impervious & storm water drains. Clean water discharge Shear stressUp arrow.
Stream powerUp arrow.
Sediment transport capacityUp arrow.
  • Degradation
  • Channel enlargement
  • Bank erosion
  • Channel successional state
  • Increased sediment load
Stream flow increase from vegetative alteration, clear cutting, land clearing, roads Shear stressUp arrow.
Stream powerUp arrow.
Magnitude of flowUp arrow.
Duration of flowsUp arrow.
  • Channel enlargement
  • Bank erosion
  • Degradation
  • Channel successional state
  • Increased sediment load
Riparian vegetation alteration (% of channel length by stream type) Bank erodibilityUp arrow.
Sediment transport capacityDown arrow
Stream powerDown arrow
Shear stressDown arrow
  • Bank erosion
  • Aggradation
  • Enlargement
  • Channel successional state
Surface disturbances (% of ground cover) and roads
  • Surface runoffUp arrow.
  • Sub-surface flow intercept (roads)Up arrow.
  • DepositionUp arrow.
  • Sediment transport capacity (aggradation)Down arrow
  • Excess scour(degradation)Up arrow.
  • Surface erosion delivered to stream
  • Road source sediment
  • Gully erosion
  • Aggradation
  • Degradation
  • Streambank erosion
Water yield - harvest & roads - add to soil water influencing slope stability
  • Surface/sub-surface hydrologyUp arrow.
  • Soil saturationUp arrow.
  • Internal strength by rootsDown arrow
  • Slope equilibriumDown arrow
Mass wasting

slump earthflowUp arrow.

debris torrentUp arrow.

sediment supply delivered to channelUp arrow.

aggradationUp arrow.

confinement channel evolution shiftsUp arrow.

enlargement (debris torrents)Up arrow.
Direct channel impacts
Shear stressUp and down arrows
Stream powerUp and down arrows
WidthUp arrow.
ConfinementUp arrow.
IncisionUp arrow.
Gully erosionUp arrow.
Bank erosionUp arrow.
Channel enlargementUp arrow.
DegradationUp arrow.
AggradationUp arrow.
Channel successional state

Figure 68
Figure 68. Example of broad level delineation of stream types at Level 1.

Figure 69
Figure 69. Example of stream type delineation (Level I) on 7-1/2' quadrangle topographic maps on the upper reaches of the Colorado and Fraser Rivers - Colorado. (Larger view in PDF format, 97kb, 1 p..)

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Figure 70
Figure 70. Example of broad level stream type delineation using aerial photography.

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