The most recent aerial photographs at a scale of at least 1:20 000, taken after logging, are used to identify channel reaches along the mainstem. A reach is a fundamental channel unit defined as a length of channel homogeneous in channel pattern, relation of valley flat to channel, and discharge (Kellerhals et al., 1976). Partitioning the channel network into distinct reaches provides a framework within which to assess channel characteristics. Identifying reaches lets you compare consistent channel types throughout the watershed.
Each reach is identified alphabetically and marked with reach breaks on the 1:20 000 scale topographic base map compiled in the WAP level 1 analysis. Guidelines to help identify common reach breaks include:
For the purposes of CAP, changes in land use (e.g., from forest to clearcut) may also be used as reach breaks. Although this criterion is arbitrary (i.e., there are no necessary changes in channel characteristics), it is useful in a land management context.
The mainstem of a watershed can consist of a variety of components, including alluvial and non-alluvial stream channels, lakes, and wetlands. The CAP evaluates changes in bed and bank characteristics of stream channels with either an alluvial channel or a non-alluvial channel with a mobile bed or erodible banks. The CAP is not appropriate for stream channels that cannot adjust their boundary in response to the fluvial transport of coarse bedload sediment (e.g., bedrock canyons, wetlands, and gullies).
The classification of stream environments, standing water bodies, and wetlands presented by the Scientific Panel for Sustainable Forest Practices in Clayoquot Sound has been modified and adapted for the CAP. This provides common terminology for all components of the drainage network and is consistent with other Forest Practices Code documents. It also provides a basis from which to identify objectively those reaches appropriate for assessment. This classification, hereafter referred to as the "drainage network classification" in the CAP, is presented in Figure 5.
Figure 5. Drainage network classification (details are presented in Appendix 3).
The basic division in the drainage network classification is between standing water (lakes and wetlands) and stream environments (erodible and non-erodible channels). The CAP applies only to stream environments, as defined in Appendix 3. The length of all mainstem stream channels is included in the final analysis of the CAP. However, only the erodible stream channels (i.e., an alluvial channel or a non-alluvial channel with a mobile bed or erodible banks) are actually assessed.
The results of the classification, recorded on Form 1, should be field-checked as part of a detailed assessment. Reach length should also be recorded on Form 1, and can be estimated along the straight-line valley axis using relatively short individual measurements on a TRIM map.
The second step in a CAP is to complete an overview analysis of channel morphology for each reach along the mainstem by observing changes in channel characteristics through time on aerial photographs. This general assessment considers any direct evidence of changes in channel morphology coincident with forestry-related activities within a watershed. The general assessment streamlines the CAP when changes in channel morphology are obvious and significant, such as a two-fold increase in bankfull width along the entire mainstem after a road-related debris slide enters the stream channel. In these cases channel disturbance is evident, and no further assessment is required.
The general assessment relies on changes in channel morphology being clearly visible on aerial photographs. Unfortunately, this situation is limited by the scale of photography relative to the size of the channel and by the amount of riparian canopy over and around the channel. For example, at the 1:15 000 scale, a 15-metre-wide stream channel appears 1 mm wide on photographs. With a dense canopy of riparian vegetation, it is often difficult to identify changes in channel morphology at this scale. If with an initial overview of the aerial photographs most of the mainstem channel is not visible or is difficult to interpret, the procedure may default to the more rigorous detailed assessment without completion of the general assessment.
The general assessment is made by comparing the most recent, largest-scale (e.g., 1:15 000 or 1:20 000) aerial photographs of the watershed with those taken just prior to logging. If the appropriate photography is not available, the procedure defaults to the detailed field assessment. Each mainstem reach adjacent to and downstream of any forestry-related activities is assessed for changes in general channel characteristics as described in Table 1.
Table 1. General channel characteristics altered by forestry-related activities
| Channel characteristic | Indication |
|---|---|
| Sediment delivery patterns | Increased landslide frequency, or an increase in gully or bank erosion with delivery to the stream channel |
| Sediment storage patterns | Any change in bar surface area (note: changes from successive observations at high and low flows are not recorded—check date and season of photography) |
| LWD jams | Any change in the size, location or position of log jams |
| Channel width | Any change in width |
| Channel pattern | Any change in pattern |
The length of channel with changes in channel morphology should be calculated for each sub-basin (as required), residual, and for the total watershed by completing Form 2. To do this:
Then an observed and a potential channel impact value (CIV) should be calculated by completing Form 3.
Table 2. Channel impact value conversion guide
There are three possible outcomes of the general assessment (see Figure 3).
Observed CIV >0.7
The observed CIV is entered into Form 8, and the final section on "Interpretation and assessment" is completed. Neither the detailed assessment nor the field assessment are required in this high-impact circumstance, although a brief outline should be provided to the multi-agency round table, explaining why a CIV >0.7 was assigned. This should describe the processes involved (e.g., landslides induced by logging) and the resulting changes in channel morphology (e.g., increased channel width and sediment storage behind new LWD jams).
Potential CIV <0.5
The potential CIV is entered into Form 8 and the section on "Interpretation and assessment" is completed. This low-impact situation permits the completion of a general assessment, with a CIV entered into the level 1 interpretation matrices, even where the entire channel downstream of any forestry-related activities cannot be resolved on the photographs. This is permissible only when the potential CIV is <0.5 (low channel impact), recognizing that the observed CIV can never exceed the potential CIV. Neither a detailed assessment nor a field assessment are required in this circumstance, although a brief outline should be provided to describe the morphological characteristics of the visible channel (if identified) downstream of any forestry-related activities.
Observed CIV <0.7 and potential CIV >0.5
Because the general assessment is subjective and many changes in channel morphology that result from forestry-related activities are subtle, general assessment is inadequate where the observed CIV is <0.7 and the potential CIV is >0.5. The general assessment is deemed inconclusive, and a more rigorous detailed assessment is required.
