Variables Influenced

Tables 7 through 9 in the RLA identify various land use activities and their influence on the variables related to potential adverse change. The list of processes/land use interrelations (Table 9) is important to revisit during this step, as this list is augmented with several RRISSC prediction methods below. The user should also review the relevant RLA inventory information for links among Table 9 variables and specific land uses or changes. For example, a reservoir located on an unstable G4 stream type helps explain the channel degradation and enlargement below the dam.

Some of the variables influenced are magnitude, duration and timing of stream flow, clear water discharge, increased shear stress, increased stream power, change in local base level, energy slope, and other interrelated process variables. It is obvious in the example that this combination of reservoir use and G4 stream type represents a high risk of both excess sediment supply as well as reduced channel stability. Specific mitigation effects on the operational hydrology of the dam would require a much more detailed assessment at the PLA. If the reservoir mentioned above was on a B2 stream type, however, the response to the change in flow and sediment would be entirely different than on a G4 stream type, requiring very little mitigation for channel stability. A list of land use activities and their influence on process variables and potential consequences, along with the corresponding RRISSC prediction methods, appears in Table 11 below.

Land Use/Activity Variables and Factors Influenced			Risk Prediction Method
Potential Change From Land Use/Activities	Processes Influenced	Potential Consequence
Stream flow decrease in magnitude, duration and timing, due to reservoirs, diversions	Shear stress Stream power Competency Sediment transport capacity	Excess sediment deposition Accelerated bank erosion Widening channel Successional state Aggradation	Figure 71
Urban areas - stream flow discharge increase due to high % impervious cover & storm water drains; clear water discharge	Shear stress Stream power Sediment transport capacity	Degradation Channel enlargement Bank erosion Channel successional state Increased sediment load	Figure 72
Stream flow increase from vegetative alteration, clear cutting, land clearing, roads	Shear stress Stream power Magnitude of flow Duration of flows	Channel enlargement Bank erosion Degradation Channel successional state Increased sediment load	Figure 73 Worksheet 2, (PDF, 36 kb, 1 p.)
Riparian vegetation alteration (% of channel length by stream type)	Bank erodibility Sediment transport capacity Stream power Shear stress	Bank erosion Aggradation Enlargement Channel successional state	Figure 74 Worksheet 3 (PDF, 119 kb, 1 p.)
Surface disturbances (% of ground cover) and roads	Surface runoff Sub-surface flow intercept (roads) Deposition Sediment transport capacity (aggradation) Excess scour(degradation)	Surface erosion delivered to stream Road source sediment Gully erosion Aggradation Degradation Streambank erosion	Figure 75 Figure 76 Figure 77 Figure 78 Table 12 (PDF, 17 kb, 1 p.) Figure 79
Water yield - harvest and roads - add to soil water influencing slope stability	Surface/sub-surface hydrology Soil saturation Internal strength by roots Slope equilibrium	Mass wasting   slump earthflow   debris torrent   sediment supply delivered to channel   aggradation   confinement channel evolution shifts   enlargement (debris torrents)	Figure 77 Figure 80 Figure 81 Figure 82
Direct channel impacts    channelization    levees    straightening    dredging	Shear stress Stream power Width Confinement Incision	Gully erosion Bank erosion Channel enlargement Degradation Aggradation Channel successional state	Worksheet 4, (PDF, 42 kb, 1 p.) Figure 74 Figure 77 Figure 78 Figure 79 Figure 83 Figure 84 Figure 85 Table 13
Channel clearing, cleaning, grubbing, large woody debris removal	Stream power Shear stress Sediment transport capacity Competence Degradation Energy dissipation	Sediment deposition Degradation Bank erosion Channel enlargement Increased sediment supply Aggradation	Figure 77 Figure 78 Figure 79 Figure 85 Figure 86

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