Forest Practices Code - Channel Assessment Procedure Guidebook
The stream channel assessment procedure (CAP) forms the level 2 analysis of the coastal and interior watershed assessment procedures (WAP). The need to complete a CAP follows from the level 1 analysis: if any hazard index (other than surface erosion) is determined to be >0.5, then a CAP is required.
The link between this level and the WAP level 1 (described in the Coastal Watershed Assessment Procedure Guidebook and the Interior Watershed Assessment Procedure Guidebook) is shown in Figure 1. Prescriptions for channel restoration, based on the results of a CAP, are given in Hogan et al. (in preparation).
Changes in stream channel characteristics occur for many reasons. The intent of the CAP is to identify disturbed channels, if they exist, in a consistent and repeatable process. The cause of the documented disturbance will be evident in many instances. However, in other cases the disturbance will not be easily attributed to a single, definitive cause such as past logging activities.
An important factor in channel sensitivity to disturbance is the relative significance of natural sediment sources compared to possible forestry-related sources. For example, in many mountainous areas, glaciers and major alpine-origin mass movement features can dominate the sediment regime. This factor should form a basis for a level 3 analysis—it is not addressed by the CAP. A level 3 analysis must be specified by a qualified geoscientist and is unique to the conditions and history of an individual watershed. Level 3 analysis considers the entire drainage network and resolves the more complex situations where hillslope, valley bottom, stream channel and land use interactions make cause-and-effect relations difficult to determine.
Because of the importance of correctly identifying the specific nature of channel impacts and the underlying fluvial and geophysical processes that produce them, the CAP should be undertaken by a geoscientist, hydrologist, forester or engineer with an appropriate background in fluvial geomorphology. [1]
The CAP analysis is based on watershed principles. Specific channel types are associated with particular zones within a watershed (Figure 2). Channels typically range from small, steep-gradient headwater streams with little or no floodplain to large, low-gradient streams flowing over wide floodplains. The morphology of each different channel type depends on its location within the watershed. Morphology is determined by the amount and timing of stream flows, the gradient of the channel and nearby hillslopes, the nature and type of sediment and debris sources, and the extent of flat valley bottoms.
Figure 1. Flow chart showing the watershed assessment procedure. Refer to either the Coastal Watershed Assessment Procedure Guidebook or the Interior Watershed Assessment Procedure Guidebook for definitions and a discussion of the procedure. Note when the CAP is initiated.
Figure 2. Relative channel size and location within a watershed (after Church 1992). Small channels: individual clasts constitute significant form elements, with pools dammed behind individual rocks or a line of rocks. Intermediate channels: channel width is typically 20–30 m with a riffle-bar-pool sequence interrupted by large woody debris (LWD) pieces and jams. Large channels: channel width is >20–30 m with a riffle-bar-pool sequence determined entirely by fluvial processes and geological constraints.
The CAP consists of both office and field work. The office work involves a comparison of the most recent aerial photographs with those taken before logging, taken at a scale of at least 1:20 000, to identify channel disturbance. The complete procedure, showing order of tasks, decision paths and links to the WAP level 1, is outlined in Figure 3. The CAP provides an initial assessment of channel impacts from which the need for further assessment can be determined. In most cases the CAP is all that will be required.
General assessment uses aerial photographs exclusively, while detailed assessment restricts their use to channels wider than approximately 20 to 30 m. The field work identifies general channel types to qualitatively measure the amount of channel disturbance. Detailed assessment prescribes field work on channels less than approximately 20 to 30 m wide.
Additional analyses should review locally relevant historical hydrological data to determine when major storms occurred in the area in relation to both the available aerial photographs and timing of the field work. The analyses should evaluate any pre-existing channel inventories or descriptions. Data of this sort may be considered "locally relevant" if it is obtained from similar hydrologic and physiographic regions, considering the morphometric characteristics of the watersheds under comparison (e.g., size, average gradient, amount of steepland and valley flat). This information is usually collected prior to commencing a level 2 assessment; it is prerequisite to the level 3.
Figure 3. Flow chart showing the channel assessment procedure. Note that the detailed assessment can be initiated without first completing the general assessment.
The five sections in this guidebook explain, step by step, how to complete the CAP. The section on "Reach classification" details how to identify and classify channel reaches. The section on "General assessment" describes the tasks in completing the CAP by analyzing aerial photographs. The section on "Detailed assessment" describes the tasks in completing the CAP through both aerial photograph and field assessments. (The field assessment methods are contained in the Channel Assessment Procedure Field Guidebook, intended to be taken into the field.) The final section on "Interpretation and assessment" incorporates results from all components of the CAP. It summarizes the results and calculates a channel impact value used in the WAP level 1. All the forms to complete the CAP are located in Appendix 1; reproduce the forms as needed.
The first step in a CAP is to divide the channel network into a series of reaches, each with consistent physical characteristics, and then use these to identify where a CAP is required. This step includes the mainstem portion of the channel network both downstream of any forestry-related activities and upstream of the point of interest (as defined in the WAP level 1).
The mainstem channel is identified in the CAP by considering the stream order at the point of interest (the highest order channel in a watershed—see Figure 4). The mainstem includes the highest order channel in the residual watershed and the highest order channel in each sub-basin with any hazard index (other than surface erosion) greater than or equal to 0.5 (see either the Coastal Watershed Assessment Procedure Guidebook or the Interior Watershed Assessment Procedure Guidebook for guidelines to help identify sub-basins and the residual watershed). Logged tributaries to the mainstem are not assessed by the CAP. In practice, the CAP assumes that the cumulative impacts of past forestry-related activities that have occurred throughout the watershed (if present), will be evident along mainstem channel reaches.
Figure 4. Method used to identify the mainstem at point of interest (POI). The mainstem channel includes the fourth order channel in the residual watershed and the third order channels in each of the three sub-basins (A, B, and C). Refer to either the Coastal Watershed Assessment Procedure Guidebook or the Interior Watershed Assessment Procedure Guidebook for guidelines to help determine stream order.
