The Kawesas Watershed Assessment

IV: Aquatic Habitat & Salmonids

Michael Pollock

Salmon are a valuable resource economically and culturally for First Nations and non-Native peoples in British Columbia. The control of salmonid population levels in freshwater environments such as the Kawesas River and its tributaries is primarily dependent on physical habitat conditions, which are driven in large part by geology and geomorphology (examined in-chapter two) and vegetation dynamics (examined in chapter three). In addition, salmon populations are dependent on aquatic insects (the subject of chapter five) during the juvenile phase of their life cycle. When they return from the ocean to spawn, salmon bring back to the ecosystem abundant marine nutrients which are crucial for various wildlife (chapter six) and human uses (chapter seven).

Forest, management, particularly timber harvesting and road construction, can influence physical features of streams, and thus the population levels of salmonids (Meehan 1991). This assessment is therefore concerned with describing the physical features of salmonid habitat and the distribution and life histories of Kawesas salmonids. The likely effects of timber harvest on salmon and their habitat are discussed, as well as measures that can be taken to minimize these effects. One focus of this study was the large woody debris (LWD) found in the river, and the estimation of the area of mature floodplain forest needed to supply the Kawesas with this material, which is crucial to maintaining the river's natural dynamics.

Large woody debris is an important physical feature of salmonid habitat in forested watersheds (Fausch 1992, Naiman 1992). LWD influences most physical factors associated with salmonid habitat, such as velocity profiles, sediment sorting, and pool formation (Swanson 1978, Keller and Swanson 1979). Therefore, the removal or disturbance of LWD in streams (e.g. during timber harvesting) can affect salmonid habitat. Even when LWD is not removed from streams, logging can result in the long-term depletion of LWD by removing future sources (Murphy and Koski 1989). Because of the importance of LWD to salmonid habitat, any analysis of habitat conditions must include an assessment of LWD stocks and future sources.

Because the Kawesas watershed has not been degraded by human activities, assessment of habitat degradation and restoration strategies, which are typical components of watershed assessment protocols, are not included in this analysis. The purpose of this chapter therefore, is to describe the distribution and abundance of salmonid populations and salmonid habitat within the Kawesas watershed. Specifically, this chapter: (1) describes the distribution of both juvenile and spawning salmonids within the watershed using surveys and historical data, (2) describes current habitat conditions and the distribution of specific habitat types, (3) estimates the current distribution of LWD within these habitats, and (4) estimates the amount of riparian forest necessary to ensure a source of LWD to the stream system in perpetuity.

Historic trends in salmonid abundance and distribution

Six species of salmonids occur in the Kawesas drainage: coho salmon (Oncorhynchus kisutch), chinook salmon (O. tshawytscha), pink salmon (O. gorbuscha), chum salmon (O. keta), cutthroat trout (O. clarki) and dolly varden (Salvelinus malma). Additionally, coastrange sculpin (Cottus aleuticus), three-spine stickleback (Gasterosteus aculeatus) and oolichan (Thaleichthys pacificus) utilize the Kawesas. No sockeye salmon (O. nerka) or steelhead (O. mykiss) are known to spawn in the Kawesas drainage, although Haisla fishers catch sockeye in Chief Mathews Bay from the end of May through August (Table 3). Since 1953, the size of the adult population has been estimated to be as high as 1,500, 200, 7,500 and 7,500 individuals for coho, chinook, pink and chum, respectively (Canadian Department of Fisheries and Oceans 1995). Information on distribution and abundance is available only for these four species.

Coho salmon

The Kawesas coho enter the watershed in August, spawn in September and October, and fry emerge the following spring. The Canadian Department of Fisheries and Oceans (DFO) began surveying the stream for escapement in 1953, but escapement counts are highly variable because of year to year differences in methodologies, time of survey, water conditions at the time of survey and changes in personnel. During 15 of the 43 years of record, salmon escapement estimates were made when there were no coho in the river, and there is no escapement record for the last 10 years (DFO 1995). Of the remaining years of record, the run size ranged from 25 to 1,500 and averaged 439 adults (SD + 129, n=18). Given the questionable escapement counts and the wide variation in escapement estimates, these data can only serve to suggest that there is a run of approximately 500–1,500 coho in the Kawesas. Accordingly, the DFO has set a target escapement of 1,000 adults.

Chinook salmon

Kawesas chinook enter the watershed in July, spawn in August and September, and fry emerge the following spring. Most of the chinook are probably ocean-type, meaning that they migrate to sea in the spring or early summer of their first year. The ratio of ocean type to stream type is strongly influenced by latitude. The Kitimat River, the closest river where race ratios are known, is 88% ocean type, and the nearby Skeena River is 52% ocean type (Healey 1991). As is the case with coho, escapement counts are highly variable and somewhat questionable. During 16 of the 43 years of record, salmon escapement estimates were made while there apparently were no chinook in the river, and there is no escapement record for an additional 12 years (DFO 1995). Of the remaining years of record, the run size ranged from 25-200 and averaged 75 adults (SD + 57, n=15). Given the questionable escapement counts and the wide variation in escapement estimates, these data serve to suggest that at a minimum, there is a run of 100-200 fall chinook in the Kawesas. The DFOO apparently believes that the run may be larger, and has set a target escapement of 500 adults.

Chum & pink salmon

Kawesas chum enter the watershed in late August, spawn in September, and emerge the following spring. Pink salmon likely follow a similar phenology. The same caveats that apply to the coho and chinook escapement counts also apply to escapement estimates of chum and pink salmon. During 17 of the 43 years of record, escapement estimates were made while there apparently were no pinks in the river, and there is no escapement record for the last 10 years (DFO 1995). Of the remaining years of record, the run size ranged from 3 to 7,500 and averaged 1,139 adults (SD + 1,932, n=16). For chum salmon, the record is slightly better. Only 6 of the 43 years of record were escapement estimates of zero made. Again there is no escapement record for the last 10 years. Of the remaining years of record, the run size ranged from 20 to 7,500 and averaged 1,081 adults (SD + 1,975, n=27). Again, given the questionable escapement counts and the wide variation in escapement estimates, these data can only serve to roughly estimate average run sizes, which are probably in the range of 1,000-5,000 adults for both pink and chum. The DFO has set a target escapement of 1,000 adults for both of these species.

Salmonid habitats

Virtually all salmonid habitat is confined to a narrow strip of low gradient terrain in the main river valley corridor. There are six general categories of aquatic habitats in the Kawesas basin that are important to salmonids: the main river channel, side channels, wall-base channels, valley wall tributaries, valley floor tributaries, and beaver ponds (Sedell 1984, Bisson 1987, Naiman 1992). These categories represent the most important types of aquatic habitat that are utilized by salmonid species in gravel-bedded rivers of the Pacific Northwest. A comprehensive description of these six habitats is given in the KWA Technical Report. We summarize the abundance, distribution, and characteristics of five of these habitat types (grouped together as off-channel habitat) in this document.

A total of 6,525 m of five types of off-channel habitat (valley floor tributaries, valley wall tributaries, side channels, wall-base channels, and beaver ponds) were surveyed for physical characteristics indicative of salmonid habitat quality. The total length of off-channel habitat within the main valley corridor was estimated from aerial photographs. An estimated 25,100 m of off-channel habitat, 6,200 m of valley floor tributaries, 9,800 m of side channels, 7,200 m of valley wall tributaries, and 1,900 m of wall-base channels in the mainstem valley are accessible to anadromous salmonids. Beaver ponds could not be accurately identified from 1:40,000 air photos. Therefore, estimates of the total amount of this habitat type are not available. It was also difficult to identify wall-base channels, because of their typically narrow widths, short length, and tendency to be hidden by forest canopies. Therefore the estimate of total wall-base channel habitat is probably a conservative figure. All surveyed habitat types consisted of greater than 50% pools. Wall-base channels and beaver ponds had particularly high proportions of pool habitat (85% and 100%, respectively).

Salmonid surveys

Juvenile salmonid habitat preferences

Over the sampling period, 490 salmonids were captured at 66 trap sites. The total catch was dominated by coho fry; 88.8 % of all salmonids and 78.7 % of the biomass consisted of coho. Therefore, this analysis is in large part an analysis of the distribution and abundance of coho fry. Juvenile salmonids exhibited distinct preferences for certain types of habitats, resulting in a wide range of biomass between each of the habitat types. Biomass per trap was highest in valley floor tributaries (111.7 g + 9.3 se) and wall-base channels (98.8 g + 21.3 se), and lowest in side channel riffles (4.1 g + 1.03 se). In general, side channels, valley wall tributaries and the main channel were low in biomass and total fish numbers, while wall-base channels, beaver ponds, and valley floor tributaries had comparatively high biomass and total numbers.

Juvenile salmonid populations and LWD

There were striking, highly significant (p < 0.001) differences between both the average site biomass and the average number of fish when comparisons were made between sites with LWD (<1 m from trap) and sites without LWD. Thirty-three sites without LWD averaged 6.0 g (+ 3.0 se) salmonid biomass, while the thirty-three sites with LWD averaged 64.7 g (+ 9.3 se) biomass, an order of magnitude difference. The number of salmon in sites without LWD averaged 1.2 (+ 0.4 se) fish, while the number of salmon in sites with LWD averaged 13.7 (+ 1.9 se) fish. There were also highly significant (p < 0.001) differences in the average biomass per fish between sites with and without LWD. Fish in sites without LWD averaged 3.53 g (+ 0.9 se), while fish in sites with LWD averaged 4.71 g (+ 0.3 se).

Spawning areas

Coho have been observed spawning as far as 25 km up the mainstem of the Kawesas (DFO 1995) and likely spawn throughout the mainstem and the low gradient sections of many of the tributaries. Spawning coho have also been observed on Coho Creek, a small stream which empties into the Kawesas estuary, approximately 0.7 km from the mouth of the Kawesas. The size of this population is unknown, but the abundance of coho fry observed rearing in the stream suggest that the run size is significant. Chinook have been observed spawning as far as 26 km up the mainstem of the Kawesas, stopping where a cascade in the mainstem forms a barrier to anadromous fish migration [DFO 1995). Chum and pink salmon have been observed spawning in the lower 12 km of mainstem, well above a series of rapids created by a debris fan less than 1 km above the junction of Cole Creek and the Kawesas River (DFO 1995). They probably also spawn in the abundant side channel habitat found in the lower reaches.

Surveys of over 6.5 km of off-channel habitat in the watershed suggests that most side channels and the larger valley floor tributaries contain habitat suitable for spawning. The mainstem also contains numerous locations where physical conditions suggest there is suitable spawning habitat for chum, pink, coho, and chinook. Undoubtedly, there are many locations that are suitable for spawning that are not used by salmonids. Very little spawning habitat is available in beaver ponds, wall-base channels, and the smaller (<2 m) valley floor tributaries, primarily because of the accumulation of fines (mostly silts) in the pools and a lack of riffles. Valley wall tributaries generally have substrates larger than 128 mm, high velocity (>1 metre per second) flows and very turbulent water, making them unattractive as spawning sites. Reaches of valley wall tributaries flowing over debris cones often contain small pockets of spawning gravel that could be utilized by anadromous salmonids.

Barriers to migration

Migratory obstacles occur 14, 15, and 25 km from the mouth of the mainstem, where there are high gradient, constrained reaches containing boulders. There is another possible barrier to migration on the mainstem approximately 1 km up from the junction with Cole Creek, where a boulder field maintained by an adjacent debris cone forms a set of rapids. However, spawning surveys indicate that populations from all four species of anadromous salmon known to utilize the drainage are able to surmount this obstacle. The first two barriers (km 14 and 15) probably obstruct the movement of chum and pink salmon, if they migrate that far upstream. The rapids at kilometre 25 may be an obstruction to chinook, coho, and (if they are present) steelhead. The entire mainstem is 26 km long, terminating where two forks join together. Both of these forks descend down waterfalls from steep hanging valleys, positively forming an obstacle to migration for all salmonids. Cole Creek also descends from a hanging valley, forming a steep chute approximately 1.2 km upstream from its junction with the Kawesas. This chute forms another barrier to salmonid migration. All other tributaries descend from steep valley walls, usually forming barriers to migration less than 500 m upstream from the junction of the tributaries with the mainstem.

Abundance and distribution of LWD in off-channel habitat

Large woody debris surveys

The amount of LWD varied considerably as a function of off-channel habitat. Wall-base channels had by far the most LWD, averaging 199 pieces/km, while beaver ponds had 99 pieces/km, valley floor tributaries contained 86 pieces/km, side channels contained 62 pieces/km and valley wall tributaries contained just 53 pieces/km. Most of the LWD in these habitats were long (>9 m) and between 0.2–1.0m dbh. Beaver ponds had a relatively high percentage of short, small diametre LWD (0.2-0.5 m), primarily because of the wood-cutting activity of beaver in these areas. Beaver ponds and wall-base channels typically had higher numbers of long (>6 m), large diametre (>0.5 m) LWD, relative to valley floor tributaries, valley wall tributaries, and side channels. This is likely a result of the stability of these habitats. Even during major floods, stream power in these habitats is probably not sufficient to entrain large pieces of LWD.

Abundance and distribution of LWD in the main channel

Large woody debris surveys

On the main channels of the basin, a total of 1,091 pieces of LWD were counted on 5,470 m of river in three sections (upper, middle, and lower). Conifers accounted for 63% (684 pieces) of the LWD, deciduous trees for 24% (265 pieces) and 13% (142 pieces) was unidentifiable.

The main channels contain an average coniferous basal area of 31.35 m² per km of channel, and a deciduous basal area of 4.25 m² per km of channel. Since the mainstem is 26 km long, and lower Cole Creek and a lower mainstem bifurcation adds another 2.4 km of main channel, the entire basal area of LWD in the main channels is approximately 890 m² of coniferous LWD and 121 m² of deciduous LWD. Since samples of coniferous riparian forests in the Kawesas valley have average basal areas of 85 m²/ha, 10.47 ha of coniferous forest was consumed to provide all the coniferous LWD currently in the main channels of the Kawesas. Estimates of the basal area of deciduous forests are not available.

Using the basal area of floodplain forest (85 m²/ha) and an estimate of the annual turnover or flux rate of LWD, the amount of riparian forest utilized by the river as a source of LWD over the course of centuries can be calculated. Studies of similar river systems (Murphy and Koski 1987, Swanson and Lienkaemper 1987) suggest that flux rates of 3–10% per year probably represent minimum rates of LWD movement through the main channels of the Kawesas, although other studies suggest flux rates may be more than 15%. In the absence of observed flux rates, these rates are the best available numbers for calculating the amount of forest consumed by the Kawesas River on an annual basis.

Using flux rates of 3%, 10%, and 15% per year, approximately 78, 261, and 392 ha, respectively, of the roughly 178 ha of riparian coniferous forest with slope <3% may be used by the river as a source of LWD every 250 years. In other words the main channels of the Kawesas may consume 44% to 220% of these forests every two and a half centuries.

If the amount of coniferous forest consumed exceeds 100% of the valley floor forests, then the disturbance cycle must be shorter than 250 years, or LWD may also be coming from forests covering debris cones and steep slopes adjacent to the river. The total area of the valley within 30 m elevation of the river is 1,077 ha, of which 403 ha is coniferous. This area serves as an approximation of the total area of potential LWD sources. Some LWD may come from slopes that are more than 30 m above the river channel, but this amount is probably not significant. For flux rates of 3%, 10%, and 15%, the river would consume, over a 250 year period, coniferous forest covering 19%, 65%, and 97% of this area, respectively.

Another method for estimating the area of riparian forest consumed by the river over the last 250 years is to calculate the total area in the river valley where vegetation has been severely disturbed. Cover maps of the Kawesas drainage provide such information, and show that 422 ha (72%) of the valley's 585 ha of floodplain forest has been severely disturbed at least once in the last 250 years. If we consider the entire valley floor (1,077 ha) including riverbed, gravel bars, deciduous forest, muskeg, and shrub communities, only 163 ha (15%) has escaped severe fluvial disturbance long enough to develop into old-growth (>250 years old) forest. These observations and the LWD standing stock in the mainstem suggest that the Kawesas has high meander and LWD flux rates. All these analyses indicate that the Kawesas probably will consume most of the valley floor coniferous forest over the next 250 years.

Discussion and recommendations

Salmonid habitat

High quality anadromous salmonid habitat is limited to the riparian corridor of the mainstem Kawesas, the lower reaches of Cole Creek and the lower reaches of Coho Creek. Steep side slopes and hanging valleys limit the availability of anadromous fish habitat to a small area relative to the size of the drainage basin. In general, streams with slopes >3% had very few salmonids, and no fish were observed in streams with gradients >10%. The majority of salmonids were observed in reaches where the slope was <1%.

All habitats had a high percentage of pool habitat, much of which was associated with LWD. The best summer rearing habitat was found in wall-base channels and valley floor tributaries. Wall-base channels had high numbers of fish and the highest biomass per fish of all habitat types. Valley floor tributaries had the highest biomass of fish, but not a particularly high average biomass per fish. Beaver ponds, side channel pools with LWD, and main channel pools with LWD were also productive salmonid habitat. Side channel riffles, valley wall tributaries, main channel sites without LWD, and side channel pools without LWD generally had very few fish. Overall, sites with LWD averaged approximately 12 times as many fish and approximately 11 times more biomass than sites without LWD.

Large woody debris

The importance of LWD for juvenile salmonid rearing in riverine and estuarine environments has been well documented (Bustard and Narver 1975, Bisson et al. 1987 and references cited therein, McMahon 1992). The preferred use of wall-base channels, floodplain tributaries, and beaver ponds by juvenile salmonids has also been documented (Sedell 1984, Peterson 1984). This report demonstrates that of the six major habitat types available, juvenile salmonids in the Kawesas also preferred wall-base channels, floodplain tributaries, and beaver ponds, and showed a strong preference for LWD in virtually all habitats.

Large woody debris in off-channel habitat

For all off-channel habitat, 30 m of forest on either side of the channel is the minimum buffer width sufficient to maintain adequate levels of LWD, as most LWD entering streams comes from within 30 m of the stream (Swanson and Lienkaemper 1987, Murphy and Koski 1989, Van Sickle and Gregory 1990, FEMAT 1992). However, many of the streams entering the Kawesas are subject to debris flows and rock slides that radically and abruptly alter their course as they flow over existing debris cones and the valley floor. Because of the tendency for these streams to migrate, 30 m is probably not a sufficient buffer width to maintain an adequate supply of LWD. A more appropriate buffer width would be a minimum of 30 m on either side of all areas where there is geologic evidence of a former streambed. In some instances even this may not be a sufficient buffer because streams may also form completely new channels as old channels are filled with debris.

Large woody debris in the main channel

The amount of riparian forest necessary to maintain current levels of LWD in mainstem rivers is not as well known, and in large part depends on the flux rate of LWD through the river. Few studies have calculated flux rates in large drainages, and no studies have determined LWD flux rates in a basin as large as the Kawesas. Available evidence suggests that rivers from drainage basins the size of the Kawesas have annual input rates of at least 10% of existing levels of LWD (Swanson and Lienkaemper 1987). Assuming that rivers are at equilibrium over hundreds of years (i.e. LWD input = LWD export), then the mainstem of the Kawesas probably turns over at least 10% of its standing stock of LWD on an annual basis. The actual value of this flux rate determines the amount of riparian forest necessary to maintain current levels of LWD. Since coniferous LWD has much higher habitat value (it lasts longer and is stronger than deciduous LWD), the amount of coniferous forest needed to be set aside to maintain current levels of LWD for the next 250 years was calculated based on various flux rates. A flux rate of 11% or greater implies that the river would consume more than half of the coniferous forest available on the valley floor. Since vegetation age-class observations suggest that the Kawesas may have consumed up to 422 hectares of a total of 585 hectares of floodplain forest over the last 250 years (Table 2), it appears likely that most of the valley floor forests (both deciduous and coniferous) will be utilized by the river as a LWD source over the next 250 years.

Areas of special concern

Because of their frequent use by salmonids as summer rearing, overwintering, and in some cases, spawning areas, it is critical that all off-channel habitat in the main valley of the Kawesas receive protection, and is adequately buffered from any logging, road-building, or other activities that could alter the flow of water, sediment, or LWD into these areas. Currently, there is not a good assessment of the quantity and location of beaver ponds and wall-base channel habitat in the watershed. Wall-base channels, in particular, are extremely productive salmonid habitats, yet hard to recognize from aerial photographs, and therefore may not receive adequate protection. A complete ground survey of the valley floor for wall-base channels should be completed before plans for roads and timber harvest areas are begun. Surveys of beaver ponds should also be completed; however, these can be done with aerial photographs (1:12,000).

The exact locations of spawning beds are not known. To determine the location of spawning beds, redd counts and spawning surveys should be made every year for several consecutive years. This information would allow land use managers to designate these sites as areas of special concern. Additionally, such surveys would provide more accurate population estimates.

Sufficient coniferous forest should be set aside to maintain current levels of coniferous LWD in the main channels of the Kawesas basin. The area of coniferous forest necessary to achieve this objective will depend on the flux rate of LWD. Until the actual flux rate can be determined, an area of coniferous forest necessary to maintain LWD at an annual flux rate of 15% should be set aside. This would amount to 392 ha of coniferous forest on the valley floor and surrounding slopes. The valley floor itself contains 403 ha of coniferous forest. These low gradient forests are the areas most likely to be consumed by the river as it meanders and thus are the most likely areas where LWD will be recruited. Therefore, these valley floor coniferous forests should receive the highest level of protection. Given what is known about valley floor forest consumption rates and the importance of coniferous LWD for salmonid rearing habitat in the Kawesas, essentially all the coniferous forests on the valley floor should be left as a future source of LWD.

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