Water Stewardship Information Sources
| Citation | Church, M and Ham, D. 2004. Atlas of the alluvial gravel-bed reach of Fraser River in the Lower Mainland showing channel changes in the period 1912-1999. UBC. |
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| Organization | UBC |
| URL | http://ibis.geog.ubc.ca/fraserriver/reports/morphatlas.pdf |
| Abstract/Description or Keywords | This atlas documents morphological changes along the alluvial gravel-bed reach of Fraser River in the Lower Mainland during the period 1912-1999 - essentially,for the duration ofthe 20 century. The reach extendsfromHunter Creek (River km 153) to the Mission gauge (River km 86), a distance of 67 km. Within thisreach the river depositsthe gravel portion of the sediment load that it carries out of the upstream canyons. Deposition of the gravel is caused by the reduction in river gradient when it enters the Lower Mainland and approaches marine base level at the Strait of Georgia.The reduction in gradientreducesthe power ofthe riverto transportsediments onward. Gravel is the first part of the river's sediment load (consisting also of sand and finer sediments) to be deposited because it is the largest material and requires the greatest power to maintain its onward transport. Because it is heavy material, it is transported along the bottom of the channel and deposited in bars within the channel, in contrast to most sand, silt and clay, which are borne in suspension by the river currents and may be deposited wherever the water flows. The river subsequently isforced to flow around the gravel bars, thereby creating an irregularly sinuous and divided (braided) channel pattern and islands. This habit is called a (Desloges and Church, 1989) because of the irregular pattern of lateral channelshifting that occurs. The resulting network of perennial, seasonal and abandoned channels represents aquatic habitat of exceptionally high quality. Hence, thisreach is ofspecial ecological value. The alluvial gravel-bed reach can be divided into five major subreaches with somewhat distinctive morphology (Table 1; Figure 1). Gradient declines by 10x between the upstream and downstream limits of the reach. This atlas further subdivides the reach into 13 subreaches for more detailed discussion. The period of study is determined by the availability of reliable documentation of river morphology. The primary materials chiefly are air photographs taken between 1928 and 1999 by federal and provincial survey agencies. The 1912 map of the river ( ) is also used as a primary reference. Reference is made to earlier information, but the quality of early surveys is not sufficient for mapped comparisons to be made. The time between mappings is approximately 10 years from 1949, but it is variable in the earlier period according to the availability of photos. In addition, the "1940" coverage is assembled from several photographic projects executed between 1938 and 1943. In the later period, actual intermap periods also vary according to the availability of the best photos, which should be obtained at low flow in order to reveal the greatest detail of the channel morphology. Distances quoted in the atlas are river kilometres above Sand Heads, the mouth of the river. The distances are based on the main channel position in 1971 and may not be exact for other dates because of changes in channel position. Furthermore, the distances may not correspond exactly with independently reckoned distancessince different lines might be adopted for measurements along the channel or channel zone. The adopted distances are standard in work reported from the UBC Department of Geography. In the atlas, descriptions are facilitated by the adoption of recognized names for river localities. These are mainly the names of the principal bar/island groups. Such features attract local names, but persistent islands and side channels also carry official names and the two may not coincide. Hence, some names used in the atlas may vary from local usage, although attempts have been made to prefer the latter. Names are given on the 1999 base map presented for each reach. In the atlas, the terms “left bank” and “right bank” are used. These terms conventionally refer to the banks of the river when the observer isfacing downstream. Hence, the “right bank” is, in general, the bank on the north side ofthe river or of an individual channel. The atlasis organized into 4 sections, each depicting two or three subreaches. Each section consists of a sequence of maps, each of which is a georeferenced air photo mosaic at 1:50 000 scale depicting the river morphology at the photo date. The channel margin from the preceding map is superimposed on each display so that channel changes in the intervening period can be observed. The earliest mosaic/map is based on the 1928 air photography (the first on the river), and the channel margins depicted on the map of the 1912 survey are superimposed on that display. In addition, at the end of each section there is a mapped comparison of the 1949 and 1999 channels, showing the net change overthe 50-year period ofmore reliable record. The maps locate cross-sections at 1 km spacing for which section surveys are variously shown for 1952, 1984 and 1999. These sections are based on channel surveys from those dates. Each section of the atlas is accompanied by a brief text summarizing the main changes observed in each reach. In those descriptions, references to specific sites are annotated by (xxA), (xxB), etc., where xx is the date of the base airphoto map (e.g., 19 ), and the letter corresponds with a letter on the airphotomap. Apeculiarity of the presentation isthat reaches,subreaches and river cross-sections are presented and discussed in reverse enumerated order (from subreach 13 to subreach 1, for example). That is because our survey numbering systems proceeds upstream from Mission gauge, but the logical order for interpretive discussion proceeds downstream,with the flow. Additional information presented in the atlas includes talweg (deep channel) position on each of the survey dates, the long profile along the talweg line at each date, and information of gravel texture through the gravel reach, collected between 1982 and 2001. It must be recognized that each of the photomaps depicts the river at a different flow, so that part of the apparent change in bar deposits observed from map to map may be due to water level changes. The superimposed channel margins, however, represent vegetation edges, so that they are consistently comparable with vegetated channel margins seen on the later map. Comparisons before 1949 are probably less accurate than later ones because the 1912 map may not be entirely reliable for features within the channel zone (in a few places distinctly unusual island features are shown). Furthermore, georeferencing of the 1928 photosmayhavebeenlesssuccessfulthanoflatersets both because of extreme distortion in those photos and because of a relative lack of identifiable reference points at that early date . Finally, the "1940" map is a collage from several dates. In interpreting the maps further, the flood history of the river (Figure 2) should be kept in view. Major floods transport much more sediment than the smaller ones and so are capable of effecting major changes on the river. The primary gauging record along the river is from the Hope gauge (Water Survey ofCanada station 08MF005; drainage area 217 000 km ) and is continuousfrom 1912. The largest floods within the period ofstudy occurred (in rank order) in 1948, 1972, 1950, 1964, 1955, 1997, 1921, 1999, 1920, 1967 and 1974. However, the highest known flow on the river occurred in 1894, before the first reliable surveys but near enough the early record to possibly have had special influence over it. Analysis of the flood records demonstrates that above average and below average floods tend to be clumped into recognizable periods. Hence, the period before 1925 was one of dominantly above-average floods while the following period to 1947 had mainly below average ones. From 1948 to 1976 floodstended to be very high (7 of the 11 highest flows occurring in this period), and then low to near the end of the record (see Figure 3). Finally,some circumstances of the sedimentation history of the river may have a significant bearing on observed river channel changes. During the 18 and early 19 centuries, the period ofrelatively cool climate known asthe “Little Ice Age” may have created conditions of increased sediment supply from certain mountain tributaries of Fraser River. Perhaps more significantly, the latter half of the 19 century witnessed major sediment disturbances along the Fraser itself and certain of its principal tributaries, including extensive placer gold digging on the river bars and terraces and the major railway building projects along the Fraser and Thompson rivers. Engineering disturbances continued until after the mid-20 century with railway and highway construction. Gravel and cobbles introduced into the river and its tributaries by these activities would require many decades to pass downstream into the Lower Mainland. River channel changesin the 20 century may in partreflect themovement and deposition ofthesematerials. |
| Information Type | report, mapping |
| Regional Watershed | Lower Fraser |
| Sub-watershed if known | Fraser River |
| Aquifer # | |
| Comments | |
| Project status | complete |
| Contact Name | Michael Church |
| Contact Email | [email protected] |