[Coastal Watershed Assessment Procedure Guidebook Table of Contents]
Appendix 2
Identification of stream orders, sub-basins, and the point of interest
in community watersheds
For many applications of the coastal watershed assessment procedure, it will be necessary to subdivide large watersheds into smaller units. This is done according to stream order and sub-basin designation. In cases where the watershed of analysis is relatively small (e.g., approximately 1000 hectares), subdividing it into smaller sub-basins will not likely be necessary.
Stream order
First-order channels are non-branching headwater channel segments. Second-order channels are those that receive only first-order channels (i.e., where two first-order channels join, the channel below the confluence is referred to as a second-order channel). Third-order channels are those where two second-order channels join; fourth-order channels are those where two third-order channels join (Figure 2-1). All stream ordering should be done on TRIM maps, where they are available.
Sub-basin identification
Dividing a watershed into sub-basins can be an exercise in creativity. Some general “rules of thumb” are included here to assist in the task.
Once subdivided, a watershed will commonly include sub-basins and face units:
- Sub-basins are delineated for all streams one order lower than the stream that contains the point of interest (POI). For example, if the POI is on a fourth-order stream, sub-basins should be identified for all third-order streams. If lower-order tributaries—in this case second- or, rarely, first-order—drain directly into the fourth-order stream upstream of the POI, they should be considered as separate sub-basins only if they are similar in size to the third-order sub-basins.
In watersheds being assessed for fishery concerns, it is most common to consider the watershed area upstream of the most upstream fish-bearing reach as a single sub-basin and to not further subdivide that sub-basin into smaller units. However, in addition, any sub-basin that discharges directly into a fish-bearing stream reach should be assessed individually.
- Face units are those hillslope areas that drain directly into the length of stream containing the POI, without any obvious route of surface flow (i.e., no first-order stream). These areas drain either by subsurface flow or by very small ephemeral (seasonal) streams. Small, lower-order tributaries and face units should be included in level 1 calculations in the residual category.
In the example given in Figure 2-1, Dome Creek is a salmon-bearing river that flows directly into the Fraser River. The POI is the confluence of Dome Creek and the Fraser River. Dome Creek at this point is fourth order. Here then, the Dome Creek watershed should be sub-divided into:
- sub-basin A
- sub-basin B
- sub-basin C
- the residual.
The residual in this case is everything outside sub-basins A, B, and C, but within the Dome Creek watershed. All of the assessment information (such as road density below the 300-m contour) should therefore be collected and compiled for these four units.
The totals for the entire watershed would be the sum of the four.
Figure 2-1. Stream ordering technique. The entire watershed is a fourth-order watershed. Sub-basins A, B, and C are third-order watersheds.
Point of interest determination for community watersheds
Community watersheds are watersheds that have a drainage area no greater than approximately 500 km², and that are licensed under the Water Act by the Ministry of Environment, Lands and Parks for waterworks or domestic water supply (refer to the Community Watersheds Guidebook). Most communities using surface water sources extract water from streams. A large number, however, extract water from lakes.
The following are guidelines for establishing the point of interest (POI) for a community watershed. Information on how to determine where water system intakes are and how to determine water use is also included. The final decision on where to locate the POI should be made at the initial round table meeting.
Point of interest determination for intakes on streams
- When there is only one community intake on a stream, the point of interest (POI) is at the water intake. If there is more than one licensed community intake, the POI is established at the lowest intake on the system.
- If one or more individual water licences exist on the stream within 100 m downstream from the licensed community water intake, a decision may be made to move the POI downstream to provide protection for the individual licences. Also, if a cluster of licences some distance downstream from the community intake meets criteria set out in example 2 below, the POI should be moved downstream. If there is more than one intake associated with the community watershed, the POI should be set at the lowest intake in the cluster.
Point of interest determination for intakes on lakes
Although the CWAP was developed to assess stream watersheds, it may also be applied to lake watersheds. Two examples of where it may be used:
- for a community intake in a very small lake (lake surface area less than 5 km²)
- for a community intake situated in a lake close (within 0.5 km) to a point where a stream enters the lake.
In the first example, it may be appropriate to set the POI at the outlet of the lake. In the second it may be appropriate to set the POI at the confluence of the stream or streams that could impact the water quality at the intake (or intakes). Deciding the lake POI will have to be undertaken on a site-specific basis. The round table coordinating committee should make the determination.
Point of interest determination for fisheries watersheds
In watersheds being assessed through the WAP for fishery concerns, it is common that two POIs are established:
- The first POI is established at the most downstream fish-bearing stream reach in the watershed. This is commonly the ocean or a lake, or at the confluence with another river.
- A second POI is established at the furthest upstream reach of fish-bearing stream. The watershed area upstream of this upper POI is commonly considered a single sub-basin for analysis, and is not usually subdivided further unless the round table considers it necessary. Any sub-basin discharging directly into the fish-bearing stream reach (i.e., between the lower and upper POIs) is considered an individual sub-basin and should be assessed separately.
Obtaining water licensing information
Information on the approximate location of water system intakes can be obtained from files or maps at the regional offices of the Ministry of Environment, Lands and Parks, Water Management Branch. Because some intakes are not located with precise survey techniques, they may have to be field checked.
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