The third step in a CAP is to complete a detailed assessment. General assessment is appropriate when channel disturbance, as seen on the aerial photographs, is both obvious and spatially extensive. However, this circumstance is infrequent. More commonly, disturbance is subtle, obscured by the riparian canopy, and distributed to varying degrees throughout the channel network. The detailed assessment evaluates the spatial extent and magnitude of mainstem channel disturbance downstream of forestry-related activities within a watershed using both aerial photography and field visits.
The detailed assessment consists of two parts. First, an aerial photograph assessment is done on all large channel morphologies within the drainage network (bankfull widths greater than approximately 20 to 30 m). Any change in channel morphology associated with large channels is evaluated directly by comparing photographs taken before and after logging.
Second, a field assessment is done on any intermediate or small channel morphologies (bankfull widths less than approximately 20 to 30 m), because they generally are not visible on the aerial photographs. The CAP combines field indicators of disturbance with a series of channel keys to determine the condition of the channel in the field. The assessment assumes the channel was stable prior to logging (the validity of this assumption should be tested for each watershed in a level 3 analysis). The field assessment is briefly described here; the complete procedure is given in the Channel Assessment Procedure Field Guidebook.
The detailed aerial photograph assessment directly observes changes in channel morphology after forestry-related activities within a watershed. The objective is to streamline the procedure where a large channel morphology is present (see Figure 2) and visible on aerial photographs. Reaches appropriate for this assessment are identified from the previously completed drainage network classification and include CA3aiii and CA3biii reaches (see Figure 5).
As in the general assessment, changes in channel morphology are identified in the detailed aerial photograph assessment by comparing the most recent and largest-scale aerial photographs taken before logging with those taken after logging. If appropriate photography is not available, the procedure defaults to field assessment. Additional series of aerial photographs should be included if available, including photographs taken during active logging operations as well as the earliest photographs after logging. If time permits, it is worthwhile to review the earliest set of aerial photographs available prior to logging, because they can indicate more completely the natural variability of the stream.
Changes in large channel morphology in response to forestry-related activities within a watershed can be generalized. Consider the consequences of an increase in discharge or bedload sediment supplied to the channel. Schumm (1977) suggests that, under these circumstances, the channel should become wider and less sinuous, with a longer meander wavelength. Church (1992) separates large channels into three phases (a bed material supply-dominated phase, a transitional phase, and a wash material supply-dominated phase). Each phase may have a range of morphological characteristics, depending on sediment supply (Figure 6).
Figure 6. Morphological types of large channels (after Church 1992). A letter under each channel diagram indicates the stability class associated with each sediment supply phase.
Each mainstem reach with a large channel morphology downstream of any forestry-related activities is assessed for changes in morphology by comparing the sequential aerial photographs. Use Figure 6 as a guide to identify the sediment supply phase and the stability class of the channel before logging (stability class is indicated by a letter under each channel). Then, changes in channel morphology are identified by referring to the post-logging photographs and identifying any changes to the following:
Figure 7. Channel patterns (Kellerhals et al. 1976). Channels generally become straighter with an increased discharge of sediment and/or water. Compare the pre- and post-logging photographs for any change in channel pattern (e.g., a change from a "sinuous" to a "straight" channel).
Figure 8. Channel islands (Kellerhals et al. 1976).
Figure 9. Channel bars (Church and Jones 1982). Bars shift from point or lateral bars to longitudinal and crescentric bars with an increasing sediment supply to the channel. Compare the pre- and post-logging photographs for any change in bar type (e.g., a change from "diagonal" to "medial" bars).
Figure 10. Lateral activity associated with large channels (Kellerhals et al. 1976). Channels may show evidence of one type of lateral stability (e.g., "avulsions" or "entrenched loop development"). Compare the pre- and post-logging photographs for any change in the relative amount of lateral instability associated with the channel.
Note that the trends shown in Figure 6 are general and that change from one stability class to another may only be completed in extreme episodes of channel disturbance. Further, the expected direction of change is within one of the three phases. Do not expect a wash material supply-dominated channel to take on the characteristics of a bed material supply-dominated channel as the result of forestry-related activities.
The results of this comparison are recorded on Form 4 as follows:
Table 3. Interpretation of the aerial photograph assessment
| Disturbance level | Guidelines for interpretation |
|---|---|
| A3 | The channel stability class has either changed or almost changed (as identified in Figure 6), and most aerial photograph indicators show the channel aggrading. |
| A2 | Most aerial photograph indicators show that the channel is aggrading, although the channel stability class (identified in Figure 6) has not yet changed. |
| A1 | Some aerial photograph indicators show the channel aggrading, although most of the indicators show that the channel remains stable. |
| S | Most aerial photograph indicators indicate stability, although some may show aggradation while others show degradation. |
| D1 | Some aerial photograph indicators show the channel degrading, although most of the indicators show that the channel remains stable. |
| D2 | The aerial photograph indicators show the channel degrading, although the channel stability class (as identified in Figure 6) has not yet changed. |
| D3 | The channel stability class has either changed or almost changed (as identified in Figure 6), and most aerial photograph indicators show the channel degrading. |
The length of channel with changes in morphology is then calculated by completing Form 5.
Observed, potential, and large channel impact values should be calculated by completing Form 6 (refer to Table 2 for conversion factors).
There are three possible outcomes of the detailed assessment (see Figure 3).
Observed CIV >0.7
The observed CIV is entered into Form 8. A field analysis is not required in this circumstance, although it may be completed if recommended by the multi-agency round table.
Potential CIV <0.5
The potential CIV is entered into Form 8. This low-impact situation permits the completion of a detailed 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 (see the general assessment for an explanation). Again, a field assessment is not required in this circumstance, although it may be completed if recommended by the multi-agency round table.
Observed CIV <0.7 and potential CIV >0.5
The length of channel with changes in large channel morphology are recorded on Form 7. With this outcome, all reaches not analyzed by the aerial photograph assessment are assessed in the field.
Field assessments are completed by following the Channel Assessment Procedure Field Guidebook. The field assessment determines the type of channel expected, given its location within the watershed and certain channel attributes, a set of field indicators of disturbance and a series of diagnostic disturbance keys.
In general, for those reaches identified by the aerial photograph assessment as requiring field visits (i.e., small and intermediate channels), the following steps are completed.
Finally, on Form 7 the results of the field assessment are combined with the results of the aerial photograph assessment and a CIV for the mainstem channel is entered into the WAP level 1 interpretation matrices. To complete Form 7:
The final step in a CAP is to summarize the results and to collate the materials used to complete the general and detailed assessments. The final CIV, representing impact on the mainstem channel upstream of the point of interest, is then entered into the WAP level 1 interpretation matrices.
To finish the CAP, first complete Form 8 by identifying the assessments undertaken, the date of the analysis, and who did it.
Finally, the CIV is entered into the WAP level 1
interpretation matrices. Recommendations presented in the WAP
level 1 are considered by the multi-agency round table and
constraints for a forest development plan are developed.
