The Kawesas Watershed Assessment

II: Terrain Analysis

Pierre Friele

Climate, rock and soil control the distribution of plants and animals; they also constrain our options for managing resources. Therefore an assessment of terrain stability and capability is fundamental to our understanding of what goods and services we can reasonably expect landscapes to produce. This portion of the Kowesas Watershed Assessment describes the dynamic surfaces — rock, soil, and water — of the Kowesas. Since the Kowesas has not been excluded from TFL 41, this section also reviews likely impacts of standard forest management practices, and suggests methods that may mitigate some of these impacts.

We prepared reconnaissance terrain, stability, and hydroriparian maps for the Kowesas watershed, as well as a sediment source map for the Kowesas River mainstem. These maps are available in ARC/INFO format from Interrain Pacific. This section summarizes a more comprehensive report in the Kowesas Watershed Assessment Technical Report that includes: descriptions of physiography, surficial materials, geomorphic processes, and methodology and reliability of terrain mapping; discussion of materials and landforms susceptible to slope instability and erosion following ground disturbance. Recommendations for ecosystem management are based on a geographic information system (GIS) analysis of maps produced from this research.

Physiography

The Kowesas lies in the central portion of the Kitimat Ranges within the Coast Mountains physiographic region (Holland 1976). Its topography is typified by well-rounded, dome-like ridges and deep, glacially-scoured valleys. Relief is dramatic, with summit elevations between 1,500 and 1,800 metres and valley bottom elevations near sea level. The tributary valleys hang 300 to 500 m above the main valley. They start in well defined cirque basins with abrupt head walls rising to ridges that are partially glacier-covered. Oversteepened, commonly deeply gullied, valley walls flank the Kowesas River, and rise to a prominent break in slope at 1,000-1,200 m elevation. Steep gully complexes in bedrock have discharged colluviums and built fans on the valley flat. Bedrock is dominated by intrusive rocks.

From the confluence of its two headwater tributaries, the Kowesas River flows 26 km to its estuary at Chief Mathews Bay. The valley bottom is generally less than 500 m across, widening to 1 km only below Cole Creek 5 km from the mouth. At the mouths of the tributary creeks and flanking the steep gully complexes, fluvial fans (river generated) and coarse colluvial fans (slope generated) grade to the floodplain. In places, the river is confined between these deposits, flowing through short blocky or bouldery rapids or longer entrenched reaches. Elsewhere, the river is unconfined on its floodplain. River gradient is about 1%.

The region is characterized by a cool, wet maritime climate. The steep mountain front forces air upward, generating intense precipitation in headwater areas during cyclonic storms. There is a pronounced fall/winter precipitation peak, with much of the annual precipitation falling as snow.

Warm temperatures from late May to the end of July produce a prolonged snowmelt freshet. During August and September, flows decline as the snowpack vanishes. Superimposed on this declining base-flow, fall storms produce runoff peaks which sometimes exceed the highest flood levels of early summer. The largest instantaneous discharge may be expected during October or November during rain-on-snow events. From November onward, most precipitation falls as snow; discharge falls to a low in February.

There is no gauging station on the Kowesas River; however, data are available for the nearby Kemano River, which is gauged above the powerhouse. These rivers are comparable because both originate in the Kitimat Ranges and most of their basins are montane. Since the Kowesas drains a smaller area above its estuary (332.5 km² vs. 528 km²), runoff response may be faster with more sharply defined flood peaks on the storm hydrograph. Using daily discharge data for the Kemano River for 1990 (Water Survey of Canada 1991) and a discharge/unit area relation, we can estimate flows for the Kowesas River for that year. Estimated mean daily discharge was 23.3 m3/sec; estimated daily discharge during the freshet was consistently above 45.6 m³/sec, with a maximum of 93.5 m³/sec on June 22; and the estimated annual maximum instantaneous discharge was 161.4 m³/sec on 9 October.

Methodology

Preliminary terrain mapping

Preliminary terrain mapping by air photo interpretation, following the Terrain Classification System for British Columbia (Howes and Kenk 1988), and Guidelines and Standards for Terrain Mapping in British Columbia (RIC 1995) was done by Pierre Friele. The entire Kowesas watershed was mapped on 1:40 000 scale air photos (BC 78131: 23-29, 32-37, 99-103, 109-113 and BC 78118: 299-305). Field checking (July 19–28) was done by Pierre Friele assisted by Stacey Brown. Checking consisted of both helicopter overview and ground checks. A two-hour helicopter reconnaissance covered the main valley and three tributary valleys. Its purpose was for general orientation and to check specific features, such as slope failures and the outlets of potential moraine-dammed lakes noted on the air photos. Ground checking was conducted along the length of the Kowesas River from the headwater confluence to the mouth. Verification of terrain mapping consisted of traversing landscape units where air photo interpretation was uncertain.

Mapping

Following field work, air photo mapping was revised. Each terrain polygon was assigned a terrain symbol, soil moisture class, and slope steepness class. Transfer of polygons from air photos to TRIM base maps (1:40,000 scale) was done by hand. Reconnaissance slope stability classes, stable (S), potentially unstable (P) and unstable (U), were interpreted for polygons within forested areas. Slope stability subclasses S-1, S-2, and S-3 are divisions of the stable (S) class. Delimitation of these subclasses, not done for standard reconnaissance slope stability mapping (Forest Practices Code 1995a), was done at the request of Interrain Pacific. Terrain features (e.g. recent debris flows, slides, rockfalls, Neoglacial moraines, rapids, etc.) and sediment sources along the Kowesas River (tributary streams, snow avalanches, eroding banks, etc.) were mapped on a terrain feature and sediment sources map (available upon request from Interrain Pacific). This map, augmented with a detailed photographic record, provides 1995 baseline data on fan surface condition, allowing ongoing monitoring of future natural or induced changes.

Riparian areas are ecologically significant because they contain more biological diversity than other habitat types and they provide linkages in the landscape. The Scientific Panel for Sustainable Forest Practices in Clayoquot Sound (1995) recommended adequate protection of the entire hydroriparian zone in order to maintain aquatic and riparian ecosystems and to provide a fully connected system of corridors in a watershed. The hydroriparian classification used for this watershed assessment is based on physical, geomorphically significant characteristics that can be determined readily during standard terrain mapping. For a detailed discussion see SPSFPCS (1995) pp. 175 and 257.

A hydroriparian map based on terrain mapping and soil pits dug on floodplains is available on request from Interrain Pacific. Extensive riparian areas on the valley flat coincide with floodplain areas. Floodplain polygons were divided to distinguish active channel, forested floodplain, and wetlands (fen, swamp). Fluvial fans and some colluvial fans were split by delineation of the active channel zone. Tributary creeks and marine shorelines were mapped as lines, with reach-breaks coinciding with polygon boundaries on the terrain map. Riparian areas not delineated at the scale of mapping include small local seepage sites and more extensive toe-slope seepage areas removed from stream channels.

Observations and discussion

The geomorphological regime

The Kowesas watershed is a geomorphically dynamic system. Nivation (surface modification by semipermanent snowbanks), alpine glaciation, and the erosion of glacial drift and gullied rock provide an abundant sediment supply. Intense precipitation, and steep slopes and channels ensure rapid transport of sediment to creeks and rivers. Mass movement events, especially debris flows, are common, streams carry abundant bedload (sediments carried and temporarily stored in the streambed) and washload (sediment carried in suspension), and as a result, creeks and rivers are commonly laterally unstable. Areas of sediment storage within the system are fans and floodplains. Evidence from two bars in the vicinity of S10 and S11 (see Map 3) suggest that large volumes of bedload are moving through the system. These bars, along an irregularly sinuous reach, are built from recent, 1–2 m accumulations of cobble gravel that partially buried young alder stands. Flood damage to the alder indicated a winter '92 flood for the upstream bar and a winter '93 flood for the downstream bar. Thus, it appears a pulse of sediment is moving through this section of the river. In two areas along the main river, downstream from the headwater confluence and downstream from S13, the river is laterally unstable and the active channel zone is wider than elsewhere. This suggests that aggradation (the process of sediment building up) may be occurring here, and that the bedload volume may exceed the river's ability to transport it during normal flood events.

Natural variations in sediment availability and channel stability have occurred through time. For example, during the late glacial period and early Holocene, when glacial material was being rapidly reworked, the volumes of sediment being transported by streams was likely much greater than at present, and streams occupied broad gravelly floodplains with braided channels. Sediment supplies subsequently declined, and vegetation became established on stabilized sections of floodplains. Recent glacial retreat from Late Neoglacial moraines may be contributing a fresh pulse of sediment to the fluvial system. This may be reflected in the large number of recent debris flows evident in the study area. Channel aggradation along Cole Creek about 4,600 years ago may have been due to a partial blockage of the channel downstream, caused either by debris build-up related to Neoglacial activity in the nearby cirque, or by a landslide or debris flows from adjacent steep east-facing slopes.

Periodic large rockfall events have continued throughout the Holocene. Wood from a buried forest just upstream from, a rockfall deposit yielded a date of about 1,200 years ago. This dates a rockfall event that blocked the Kowesas River and formed a small impoundment upstream, killing the trees.

Wood, both standing and downed, plays an important role in channel stability and thus in the maintenance of floodplain riparian habitats. On fans with laterally unstable creeks, standing and downed wood act to form small dams that trap and temporarily immobilize sediment, thereby influencing the amount of sediment that enters the river during flood events. Debris flows, snow avalanches, bank erosion, and channel avulsions (sudden stream course changes) can all introduce large woody debris (LWD) into the active channel. Although logs can form jams which might cause avulsions during flood events, once channel shifts have occurred, those same jams act to protect new backchannel habitats from scour. Thus LWD on the floodplain helps to regulate lateral instability, and it is essential in providing the instream backeddies and quiet backchannel areas that are important habitats for anadromous fishes.

TABLE 1: Implications of Slope Stability Class for Forest Management
Reconnaissance Slope Stability Class	Interpretation
S-1	No slope stability problems exist. Disturbance of floodplain surfaces could initiate some bank and channel erosion.
S-2	Very low likelihood of slope failure following timber harvesting. Expect minor production of fines in ditch lines and disturbed soils. Disturbance of streams could aggravate lateral instability and erosion of fan surfaces.
S-3	Low likelihood of slope failure following timber harvesting. Water management is critical. Expect problems with water channeled down roads and ditches and disturbed areas. Disturbance of colluvial cones and fans could lead to erosion, with accompanying sediment inputs to stream channels.
P	Expected to contain areas where there is moderate likelihood of slope failure following timber harvesting. A field inspection of these areas is to be made by a qualified terrain specialist prior to development to assess slope stability.
U	Natural slope failures are initiated in these areas. A field inspection of these areas is to be made by a qualified terrain specialist prior to development to assess slope stability.

Silvicultural concerns

Numerous geomorphological features in the Kowesas merit special attention before any development activity can be planned. Map 4 summarizes six landscape maps from the KWA Technical Report, and presents a composite picture of geomorphological sensitivity to anthropogenic (human-caused) disturbance. Blocky talus slopes and cones lack sandy matrix material at the surface. Thus they do not have a good rooting medium and are subject to water deficit. Trees growing on these sites often cling to large blocks and are largely sustained by thin organic soil that is easily damaged during yarding. As a result of soil disturbance, reforestation of blocky sites following logging may be problematic. Debris flow fans generally contain more sandy material at the surface. In the Kowesas area, fans may be more blocky near the apex, with proportionally more sandy substrate downslope. However, winnowing of the fan surfaces by streams may leave a boulder lag on these sites, deficient in rooting medium and susceptible to drought. Thus, like talus, winnowed fan surfaces could be difficult to reforest following logging. As a result, the regeneration potential of talus and debris flow fans should be examined in detail as part of any development plan. Sites with potential reforestation problems such as blocky talus slopes and cones, and debris flow fans occupy at least 12% of the productive forest, and 22% of the low-elevation forested area (within 100m of the valley floor). In addition, coarse textured materials are found in 22% and 34% of these two areas, respectively.

The Kowesas watershed is a high snowfall area and snow avalanches are common. Avalanche paths are numerous along the Kowesas River valley and spring avalanches would present a hazard to logging operations. The narrowness of the Kowesas valley and the numerous slide paths in some areas increase the hazard. Where slide paths end in mature forest it is recommended that forested buffers be left at the end of the runout zone to protect against an increase in the runout area and to provide natural protection for forestry infrastructure and logging operations. Avalanche prone areas occur in 66% and 48% of the Kowesas productive forest and low elevation forest, respectively. Also, steep and moderately steep slopes may become prone to avalanche initiation once trees are removed. Rapid snow creep and snowslides may inhibit regeneration of these sites. This possibility should be addressed in forest management plans. Slope class 4 and 5 areas within productive and low elevation forests indicate that 70% and 42% of these areas may become prone to avalanche initiation once trees are removed.

Mineral soil on the valley sides is thin and discontinuous in many areas. It is the accumulated organic matter (folisols) that provides much of the rooting medium and nutrient base for forest growth. Forest harvesting systems should be designed with soil conservation and site regeneration as primary considerations; conventional clearcutting methods are often very destructive to thin organic soils. Steep sites with potentially thin soils occupy 45% and 9% of productive and low elevation forests, respectively.

Floodplains present a particular set of silvicultural problems that require consideration (see Beaudry et al. 1990). Microtopography and the hydrologic regime of floodplain surfaces limit the success of planted seedlings. The present distribution of forest stands may not represent that which can be restocked after logging, because many trees are growing on tip-mounds and nurse logs. An assessment of flood depth and duration, and the development of a flood hazard map for the floodplain should be considered. Forested wetlands (swamps) occupy 370 ha (16% of the low elevation forest) and might present problems for regeneration because of high water tables or seasonal flooding. Other environmentally sensitive wetlands at low elevations include marshes (23 ha), fens (7 ha), shrub-carrs (34 ha), and bogs (4 ha), totaling 68 ha, and occupying about 3% of the low elevation area. A second concern relates to floodplain erosion rates and channel stability. Restocking of sites that may be consumed by bank erosion is a loss of investment. Floodplain erosion and deposition maps should be developed by forest managers to help predict areas of future erosion. Areas of river channel instability, bank erosion, recent channel avulsions, and large bank failures occur along much of the Kowesas' length and indicate the prevalence of areas of potential erosion.

Slope stability and erosion concerns

Reconnaissance slope stability mapping is intended for use in long range planning, for establishing operability lines and annual allowable cut (AAC) netdowns. Areas that are identified as unstable (U) should not be accessed conventionally. A large proportion of these areas should be removed from the AAC. Where management activities are planned for unstable or potentially unstable polygons, then field assessments by a slope specialist must be made (FPC 1995).

Much of the unstable (U) terrain in the Kowesas area lies on extremely steep slopes, including rock walls and gully systems. Potentially unstable (P) terrain is made up of steep and moderately steep slopes underlain by bedrock, which may have a thin mantle of till and colluvium. Although not apparently unstable, there is a moderate likelihood that forestry activities would increase debris slide frequencies. Roads must be constructed without sidecasting: full bench construction with end hauling would probably be necessary. Runoff must be properly routed with all natural drainage lines maintained. Areas with stability classes U and P occupy 50% and 49% of the forested area and low elevation forest, respectively.

Specific problem areas for forest management are: (i) the large collapsing banks along the S13 fan; ii) areas of fine glaciolacustrine sediments; and (iii) road access into hanging valleys. Aggravation of bank instability near the S13 fan could lead to further debris flow activity on the fan. Road construction or excessive disturbance in areas of fine sediment could lead to severe erosion problems and inputs of fine sediment into hanging valleys would require road construction on steep slopes above confined stream channels with steep longitudinal gradients. Road-induced slope failures could block channels and generate larger debris flows downstream.

The removal of trees and the necessity of roads on fans could aggravate channel instability and lead to increased erosion, which in turn could increase peak flows and sediment inputs into the main channel (Jones and Grant 1996). Colluvial fans subject to debris flow activity and fluvial fans subject to braiding should be managed carefully to minimize channel disturbance.

Road engineering

From a geomorphic perspective, of all forest management activities, the construction of logging roads is recognized as having the greatest potential impact on the landscape (e.g. Reid and Dunne 1984, Sauder et al. 1987). Roads should not be built through "U" polygons, and roads that cross "P" polygons must be checked by a terrain specialist. In general, it is recommended that roads be kept off slopes >60%. Refer to Forest Practices Code (1995b) for a detailed account of good logging road engineering practices.

Any logging road system proposed for the Kowesas valley would cross many colluvial fans, and roads would have to cross the Kowesas River more than once. In general, roads should cross fans as near as possible to their apex, where the channel is most confined and stable. Bridging structures across creeks can become dams during debris flow events, and burst dams can lead to downstream sediment floods. "Squamish" culverts or fords are generally most appropriate at these sites. Selection of bridge locations along the Kowesas River would have to be made considering channel stability and the possibility of extreme flood events. Bridge abutments located on the floodplain are likely to be washed out or isolated by bank erosion or channel avulsion. Areas where the river is confined are better bridging locations. Finally, flood surges on the river caused by large debris flows entering confined or partially confined reaches upstream, could far exceed flood levels resulting from normal hydrologic events. The main stem of the Kowesas is confined to a narrow channel for more than a kilometre approximately 17km upstream of the river mouth. Large debris flow events into this reach could cause flood surges. Such events could easily wash out bridges.

The design of engineering structures for flood prone areas in the Kowesas watershed is hampered by the lack of stream gauging data. Furthermore, the steep mountainous character of the valley, with most of its catchments at higher elevations subject to intense and localized storms, leads to rapid runoff and an unpredictable flood regime.

Channel stability and floodplain riparian habitat

In the undeveloped Kowesas watershed there is a balance between the natural frequency and magnitude of events in the present geomorphic regime and the habitats supported there. Careful layout and construction of roads and bridges is required to avoid increasing the frequency of slope failures and the magnitude of sediment inputs to streams. Increased erosion and sediment inputs could lead to channel aggradation and lateral instability. The end result would be loss of floodplain wetland habitats necessary for the sustained production of anadromous fishes, especially Coho salmon. Standing and downed wood should be maintained in strategically located preserves for the stabilization of channels on fans, the protection of backchannel areas, and the long term supply of LWD to the active channel.

About Get Data Links

 

Find your watershed

You are here:
Inforain.org » Research » The Kawesas Watershed Assessment » Chp II. Terrain Analysis