Elk River Channel Analysis
Setting The Elk River is located in the Rocky Mountains in south eastern British Columbia (Figure 1). The upper river drains a pair of small glacier-fed lakes (Upper and Lower Elk Lakes) that are situated at approximately 1720 and 1750 m elevation. From Lower Elk Lake, the river flows south and southwest for a distance of 170 km before draining into Lake Koocanusa at an elevation of 748 m. The shifts between south and southwest valley alignment are controlled by regional geologic structure (Holland 1976). The watershed area of Elk River is 4450 km2. The largest tributary sub-basins are Fording River (620 km2), Michel Creek (646 km2), and Wigwam River (737 km2).
The central part of the Elk River watershed consists of a geologic feature known as Fernie Basin (Holland 1976). Fernie Basin is a coal-bearing unit of sedimentary rocks (where “basin” refers to an ancient depositional environment). The contemporary topography of Fernie Basin consists of the central 100 km of the Elk River valley with its coal-bearing ridges on either side of the valley upstream of Sparwood and the broad upland to the east of Elk River downstream of Sparwood. The ridges and uplands of Fernie Basin are characterized by moderate slopes, except for locally steeper slopes along deeply incised tributaries of Elk River, Fording River, and Michel Creek. Maximum elevations on the ridges and uplands are around 1800 to 2200 m, whereas the Elk River valley bottom lies at 950 to 1400 m elevation in this section.
Flanking Fernie Basin to the east and west are the Front Ranges of the Rocky Mountains. These consist mainly of overthrust, southwest-dipping limestone beds that form parallel ridges with moderate dip slopes facing southwest and steeper scarp slopes facing northeast. Typical ridge crest elevations range from 2500 to 2900 m. The highest mountain peaks occur at the upper (north) end of the watershed in the vicinity of the Elk Lakes. Mount Joffre reaches 3450 m and is one of the highest peaks in the Canadian Rockies. Several other nearby peaks exceed 3000 m elevation. These peaks surrounding the Elk Lakes support the only contemporary glaciers in the watershed.
In the past, continental glaciations filled Elk River and tributary valleys with ice to an elevation of 2000 m. Both glacial erosion processes produced abundant sediment during deglaciation, which filled the river valley bottom with glaciofluvial and glaciolacustrine deposits. Contemporary hillslope processes and tributary channel incision continue to deliver sediment to the Elk River valley.
River Profile The longitudinal profile of the Elk River valley is shown in Figure B.1, based on 100-ft (30 m) elevation contours on 1:50,000 scale topographic maps. The valley gradient gradually declines from approximately 0.7% below Lower Elk Lake to 0.2% above Elko. The most marked feature along the profile is an abrupt increase in gradient below Elko as the river cuts through a canyon as it drops to the Rocky Mountain Trench and Lake Koocanusa.
A more subtle feature of the profile is the irregularity in valley gradient upstream of the Fording River confluence compared to the more smoothly decreasing gradient downstream. Upstream of Fording River, Elk River probably lacks sufficient discharge to grade its profile, so relict glacial deposits and contemporary tributary and hillslope deposits are reflected in the profile. Downstream of Fording River, Elk River is capable of transporting a larger sediment load and has graded its profile to a more typical alluvial slope.
The longitudinal profile of the Elk River valley for the alluvial section between Fording River and Elko is shown in Figure B.2. The profile indicates a gradually decreasing gradient from approximately 0.48% below Fording River to 0.24% above Elko. However, a slight profile inflection occurs midway along this section. An idealized profile – generated by fitting a quadratic function to the gradient between Fording River and Elko – illustrates the deviation of the actual profile from the idealized case. The actual profile is elevated by approximately 4 m above the idealized case, for a valley length of approximately 10 km extending upstream from Fernie. This suggests that the reach upstream of Fernie has experienced aggradation, and/or the reach downstream of Fernie has experienced degradation in the past.
Further insight can be gained from a topographic analysis of local fluvial features in the Fernie area. Figure B.3 presents the longitudinal profiles of several terrace and floodplain features relative to the valley axis based on 1-m contours on the 1:5000 scale floodplain maps (MOE 1979). Upstream of the North Bridge, the left- and right-bank floodplains form a single plane, but the potential flood inundation is more extensive on the left bank (facing downstream) as the contemporary river channel traverses the valley bottom from left to right, resulting in a lower floodplain on the left river bank.
The floodplain features upstream of North Bridge also lie on a plane (0.35% valley gradient) with features further downstream, except that the downstream features are less extensively inundated by floods, indicating a relatively higher position with respect to the flood profile. A distinct set of lower floodplain features (on a parallel plane) exists downstream of West Bridge. These lower floodplain features, which lie approximately 1.5 m lower than the higher floodplain/terrace features, suggest that the river has down- cut at some point in the past leaving the former floodplain at a relatively higher position, and forming a new lower floodplain. This supposition is supported by the presence of exposed glaciolacustrine silt deposits in the riverbed at the Riverside Resort and further downstream. The presence of this non-alluvial material indicates that the riverbed in this reach cannot be aggrading, otherwise the bed would be covered with river gravels. The presence of mature conifer trees on the lower floodplains indicates that the downcutting and floodplain formation is not a recent occurrence, and likely pre-dates river management activities at Fernie.
Valley geometry in the Fernie area is presented in Figure B.4. Valley width refers to the width of the valley bottom, including terraces 5 to 10 m above the floodplain, measured perpendicular to the valley axis. The figure illustrates the abrupt valley constriction from around 1400 m wide to 400 m wide that occurs downstream of Coal Creek. This point coincides with a shift in valley alignment from southwest to south, marking the transition to a different geological structure. It is probable that the inflection in the Elk River valley profile reflects an accumulation of excess glaciofluvial sediment at the valley constriction during glacial recession, and that the channel downcutting below Coal Creek followed shortly thereafter in response to the locally steeper valley gradient.
Historical Channel Analysis Historical channel stability at Fernie has been examined by reference to a sequence of aerial photographs taken in 1945, 1962, 1975, 1994, 1995, and 2004. Channel bankline planimetry for each of the photo dates is presented in Figure B.5, overlaid on the 2004 photos. Table B.1 summarizes active channel width and channel zone width in each year. Active channel width includes the water surface and unvegetated gravel bars, and effectively defines that part of the channel experiencing active sediment transport. The channel zone (delimited in Figure B.5) includes areas of young and mature vegetation, including channel islands, within a set of outer channel banks.
Historical changes in channel width and stability are discussed below, with reference to historical trends in Elk River flood magnitudes, human modifications to the river and floodplain, and historical river surveys. An analysis of Water Survey of Canada (WSC) records indicates the following historical trends in Elk River flood magnitudes. From 1914 to 1945, flood magnitudes were dominantly less than the long-term average; from 1946 to 1976, floods were dominantly above average; and from 1977 to 2005, floods were dominantly below average (despite the occurrence of the flood of record in 1995).
A history of river management activities affecting and responding to channel instability was presented by Talbot and Woods (1983), and updated by Boyer (1992). A series of channel cross-section surveys taken since 1975 has been analyzed in this study for vertical channel stability. A specific-discharge analysis of WSC 08NK002 Elk River at Fernie gauge data compares water levels associated with specific discharges over the period of record as a check for vertical instability or changes in channel geometry.
In 1945, some diking and gravel removals had reportedly been carried out by this time, but these activities are not evident in the photographic record. The Elk River channel, extending roughly 2.3 km upstream of Fernie, had an average active channel width of 180 m. By comparison, the width of the channel zone reflecting channel activity within the previous few decades averaged 302 m.
Downstream, the river channel had an active width of 86 m in the reach adjacent to Fernie and West Fernie, reflecting confinement by terraces and hillslopes in the narrowing valley. Further downstream below Coal Creek, the active channel width was even narrower (64 m) with little evidence of recent lateral shifting, reflecting the entrenched nature of the river in that reach. Channel zone width in the two downstream reaches was only modestly greater than active channel width (see Table B.1), reflecting the comparative lack of prominent channel islands.
By 1962, the two existing highway bridges had been built, and the river channel between the bridges had been artificially narrowed by construction of a longitudinal berm on the left bank. In addition a spur dike had been constructed below Coal Creek in an effort to align the flow towards the left bank. The channel upstream of North Bridge changed markedly between 1945 and 1962. The average width of the active channel increased to 283 m, along with localized lateral shifting of up to 500 m of the active channel banks. Much of the widening occurred by the erosion of a large island near the left bank. Channel zone width increased by 16 m to 318 m because of additional erosion along outer channel banks. Downstream of North Bridge, the artificial constriction narrowed the channel width by an average of 11 m (to 77 m) in the 2.9 km long reach extending to Coal Creek. Below Coal Creek, active channel width increased by 35 m and channel zone width increased by 27 m.
The channel changes observed upstream of North Bridge between 1945 and 1962 were likely due to the onset of a period of dominantly larger floods, possibly compounded by the construction of North Bridge and channel confinement downstream of the bridge. The series of above-average floods eroded channel banks, stripped vegetation from bars and transported above-average quantities of bed material. The channel constrictions (bridge and berm) may have increased the quantity of bed material deposited upstream of the bridge during this period of increased channel activity.
In the aerial photographs taken from 1975 onward, the Elk River channel has experienced a gradual narrowing and stabilization over time (Table B.1). Channel narrowing occurs through the revegetation of bars which are incorporated into the floodplain. This pattern is consistent with a period of mostly below-average flood flows from the mid-1970s to the present. Although the 1995 flood did cause some channel widening (compare 1994 and 1995 channel widths in Table B.1), the magnitude of the channel changes in that single flood were far less than those observed between 1945 and 1962.
By 2004, channel width had returned to 1994 conditions. Channel surveys taken since 1975 have indicated no significant changes in channel bed elevation over time. WSC 08NK002 Elk River at Fernie was established in 1970, relocated with a new rating curve generated in 1978, and again in 2003. Specific-discharge analysis of this gauge shows no clear trend in water levels associated with specific discharges since 1970. Apparently, any changes associated with the period of above-average flood magnitudes, and the bridge construction and channel confinement, had equilibrated by the mid-1970s.
The reduction in channel width has been most significant in the reach upstream of North Bridge (Table B.1). In 1962, the channel zone width was 318 m, with an active channel width of 283 m. By 2004, following three decades of below-average flood magnitudes, the channel zone has decreased to 179 m, with an active channel width of 116 m. Bank protection works along the golf course on the left bank, and the highway embankment along the right bank, have encroached into the former channel zone (1945 and 1962) in some places. These features now define a maximum potential channel zone not much wider than the maximum observed channel zone width in 1962 (Figure B.6). Bank protection works and the highway have played a limited role in directly narrowing the channel zone. Rather, these channel zone encroachments have mainly taken place following natural floodplain reconstruction. However, further encroachments could limit potential channel zone re-expansion in a future period of above-average flood magnitudes.
During the period of channel narrowing, the river has continued to deposit gravel, erode its banks, and shift laterally, but most of the channel activity has occurred in the form of thalweg shifting and bar rearrangement within the confines of the channel zone. The recent channel alignment upstream of North Bridge (a fairly straight thalweg near the left bank floodplain) does not favour significant local channel shifting. This circumstance occurs because sediment can be exchanged through the reach from bar to bar without creating large ‘choke’ points, where the channel is forced to avulse to maintain flow conveyance.
Construction of the golf course dike and bank protection on the left bank have helped maintain this alignment, and may have reduced the impacts of the large 1995 flood relative to observed changes between 1945 and 1962 when the channel course was more sinuous (Figure B.5). However, the bank protection is coming under increasing attack at certain sites as the main flow alignment shifts in response to localized sediment deposition on bar-riffle features which may eventually undermine the bank protection, and increase the rate of lateral channel shifting.