Appendix 3.

Drainage network classification

This appendix includes a modified version of Appendix 11 in the fifth report of the Scientific Panel for Sustainable Forest Practices in Clayoquot Sound, Sustainable Ecosystem Management in Clayoquot Sound: Planning and Practices (Anon, 1995). Several physical dimensions have been altered to be consistent with other Forest Practices Code documents. The classification units changed here are intended to avoid some of the operational difficulties encountered in the field. Wherever possible, the original definitions used in the Clayoquot Sound report are retained.

Bases for classification
(from the Clayoquot Sound report 5)

The most basic division is defined by the nature of the water body, because this fundamentally determines the nature of the associated ecosystems. Lotic (streams), lentic (standing fresh water), and marine are the basic units; the latter is not considered here.

Within stream channels, the most basic division is between erodible and non-erodible channels. Erodible channels flow through either their own alluvium deposits or materials erodible by flowing water, such as lacustrine, marine and certain colluvium deposits. Non-erodible channels are non-alluvial channels — those flowing on bedrock or on sediments not normally eroded by the contemporary stream flow. This corresponds with the criterion of confinement, because an unconfined channel always flows with at least one erodible (alluvial) bank. The next most important criterion is stream gradient, because this determines important aspects of fluvial processes and morphology. A third criterion is entrenchment; entrenched channels are confined within fluvially eroded gullies or valleys of some depth. A final criterion is stream size, which influences some of the physical and biological processes.

Within the class of erodible stream channels, the definition of a "floodplain" presents some difficulty. The generic definition — the surface of a body of sediment deposited by the stream — ignores whether a change of stream regime has led to degradation and consequent development of a terrace (which is not subject to inundation). The difference is ecologically significant. A definition based on the possibility of inundation occurring needs some qualification about the frequency of inundation, but it is not practically applicable in terrain analysis procedures. The best discriminators probably are the presence of indicator plant species in the understorey and immature cumulic soils, which can be decided definitively at the stage of field checking. Adopting such discriminators requires local guidelines.

Lentic freshwater environments are divided according to whether the environment is permanently open water more than 1 m deep (a lake) or is a wetland. A second distinction is the nature of the lacustrine ecosystem: oligotrophic lakes have relatively poor nutrient status. Wetlands are further classified as fen, marsh, swamp, or bog. Another criterion is water body size. As in streams, this determines some of the physical and biological processes between the water body and adjacent land.

Classification

C — Stream channels

A. Erodible channels

An erodible channel has a flanking floodplain, including estuarine channels in deltas, and alluvial fans (also called fluvial fans):

1. gradient less than 8 per cent

(i) channel width less than 1.5 m

(ii) channel width between 1.5 m and 20 m

(iii) channel width greater than 20 m

Notes: Channels with gradient less than 8 per cent (4.6°) have primary morphological units consisting of pools separated by riffles or extended rapids. Anadromous salmonids are found in these channels. Channels with gradients greater than 8 per cent, up to 20 per cent (5-11.3°), have step-pool morphology. Resident fish may be present.

2. gradient greater than 8 per cent

(i)-(iii) width criteria as in Al.

Operational rules:

1. A channel with one erodible bank and one non-erodible bank is classified as erodible. The bed need not be erodible, but a non-erodible bed with contemporaneously active erodible bank will be rare.

2. A channel with non-erodible banks but erodible bed is classified as erodible. The intent is to identify channels that can be disturbed by forestry activities. Channels that clearly cannot be disturbed are non-erodible. Those that clearly can or could undergo changes (step-pool sequences) are erodible.

3. If a reach is classified as erodible, but a non-erodible segment is encountered along the reach, the reach class will not change if the non-erodible segment is <1W_b. If the non-erodible channel extends for between 1 and 3 bankfull widths, the segment is noted, but the reach class remains erodible. If the segment extends for >3W_b, then the reach class is changed to non-erodible for the extent of the segment.

4. In Type 1 (C1), many channels less than 1.5 m wide will be secondary channels on floodplains, which will be incorporated into the unit defined by the main channel. Type 2 (C2) will usually be alluvial fans; in Type 2 (C2), width criterion (iii) is rare.

B. Non-erodible channels

1. gradient less than 8 per cent

a) not entrenched

(i)-(iii) width criteria as in Al.

b) entrenched

(i)-(iii) width criteria as in Al.

Notes: These may include fairly large channels that have degraded and now flow between terraced banks on lag armour (unconsolidated material — typically cobbles or boulders — that the stream cannot move and that is not alluvial in the current regime). An entrenched channel, as the result of fluvial erosion, is continuously confined within banks sufficiently high that overflow may not occur. Gullies, ravines, and bedrock gorges are typical entrenchment landforms.

2. gradient in the range 8-20 per cent

a) not entrenched

(i)-(iii) width criteria as in A1.

b) entrenched

(i)-(iii) width criteria as in Al.

Notes: Class (a) streams will principally have steep alluvial fans. In this gradient range, width class (iii) streams probably are non-existent. Most debris flows stop in this gradient range.

3. gradient greater than 20 per cent

a) not entrenched

(i) seasonal or perennial

(ii) ephemeral

b) entrenched

Notes: Although streams in class (3) are steep, they maintain water quality downstream and serve as animal travel routes and the site of riparian herbs and shrubs-including some with otherwise limited distribution. Fish are not normally present. Gradient is usually bedrock-controlled. In (a), one will classify mainly seasonal to ephemeral rills on hillsides. In (b), one will classify mainly gullies but could include sizable rivers cascading down bedrock-controlled channels from hanging valleys. Stream width is usually less than 1.5 m. Gully floor width is more significant than channel width, but usually will not be critical for processes. Debris flows may start and will be maintained on these gradients. An upper limit for stream channel gradients (other than cascades and waterfalls on bedrock) is 60 per cent.

Operational rules:

1. Both channel banks and the bed must be non-erodible for the stream to be classed as such. If one bank is erodible (alluvial) and one bank non-erodible, or the banks are non-erodible but the bed is erodible, then the reach is classified as erodible.

2. If a reach is classified as non-erodible, but an erodible segment is encountered along the reach, it will not change the reach class if the erodible segment is <1W_b. If the erodible channel extends for between 1 and 3 bankfull widths, the segment will be noted but the reach class remains non-erodible. If the segment extends for >3W_b, then the reach class is changed to erodible for its extent.