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Lowering of local base flow level through channel incision becomes a major adverse impact to stream stability, sediment supply and water resource uses. It also limits riparian vegetation's usefulness in stream recovery, as the normal high flows are often still below the rooting depth, generating less root mass and leading to accelerated streambank erosion. Water tables are lowered, land productivity can decrease, land loss is increased due to streambank erosion, and tributaries are over-steepened, causing headward advancement of the incision. Excess sediment is produced downstream of degrading channels, leading to further channel instability and loss of fish habitat and other related values. Degradation can often be created by contraction scour below culverts or narrow bridges, thus some road crossings can be potential contributors to degradation. The information required to complete this RRISSC step includes:

a) Example of nine scenarios of various stream channel successional stages (Figure 42, PDF 94 kb, 1 p.), and the risk ratings obtained (Table 13)
b) Stream type
c) Rural/non-urban flow increase by stream type (Figure 73)
d) Urban flow increase (Figure 72)
e) Streamflow increase due to reservoirs, clear water discharge, ramping flows, etc. (Figure 92)
f) Roads, drainageway crossing designs (potential base level shifts)
g) In-channel mining associated with base level shifts (Figure 83)
h) Summary from direct channel impacts (straightening, channelization, dredging) from risk rating summary (Figure 96)

The overall risk rating for degradation potential is accomplished by summarizing the risk rating relations from the above list in Worksheet 4 (xls, 28 kb), then adding results to the compiled information in Worksheets 5a & b and Worksheet 6.

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Worksheet 5a: Printable PDF (10 kb, 1 p.) | Excel Spreadsheet (27 kb)
Worksheet 5b: Printable PDF (9 kb, 1 p.) | Excel Spreadsheet (35 kb)
Worksheet 6: Printable PDF (8 kb, 1 p.) | Excel Spreadsheet (21 kb)

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