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I I I I S+ Q - Qso Dso 0 Water diversion from stream QO Qs+ Dso 0 Increased watershed sedimeDt yield QO Qs 0/ + Dso + Debris loading to stream S - Q+ Qso Dso 0 Water diversion to stream QO Qs- 0 Reservoir sedimentation with clear Dso water releases. Q+ -/0 Dso - Qs U rbanizatioD of watershed I I INSTABll.ITY HAZARDS. ,I I I The method identifies three groups of geomorphic hazards within a stream corridor. Hazard groups 1 and 2 (see table 1) are reach scale geomorphic processes that characterize a panicular stream environment, These processes create the chance that over the long-term a river instability may result from natural events or man-caused activities. Hazard group 3 results from hydraulic conditions occurring locally such as at a particular property or stream channel feature. I I I II I I I I I I I HAZARD GROUP 1. HAZARD GROUP 2. HAZARD GROUP 3. Existing river instability Potential for induced instability Local scour or sedimentation Reach Scale At-Site The method deals with reach scale effectS in a different manner than local effects, First, we assume that given an unaltered stream environment the general channel pattern and average geometry can be predicted using the empirical geomorphic relationships and associated constraints. These relationships then define the geomorphically active portion of the river valley. We also assume that the river corridor could be managed to have zero geomorphic hazard. This becomes the theoretical baseline condition. Second, we recognize that the probability of damage from a geomorphic hazard is due to encroachment into the geomorphically active portion of the river valley. Encroachment can occur in one of two ways: either as intrusion into the active area, or due to an alteration of the baseline that induces instability. Induced instability is causes by the alteration of sediment or water discharge to the reach, or the alteration of channel geometry or gradient. Local stream stability is based on observation of site-specific conditions in the stream. Local disturbance are major alterations in channel geometry and alignment at a site that fall beyond the normal baseline for a stream classification. Local disturbances due to natural causes such as tributary debris flows, floating debris accumulations can create risk. Many modern infrastructure or development projects (transportation routes, hardening of stream banks at select locations, in- stream mining) can also pose a similar risk. Since most stream types are dynamic, local stream stability must always be taken in context of the baseline stability of the geomorphically active area. 13