Fraser River Gravel Reach Studies
UBC Department of Geography

Home
Overview
Study Area
Data
Reports and Publications
Air Photos
Email for Information
Currently(2000) the Fraser River Gravel Reach Study is comprised of 6 basic elements. 
A summary of each element appears below.

These six elements are as follows: 
Element 1. -- Database Development
Element 2. -- Gravel Removal
Element 3. -- Sediment Budget
Element 4. -- Channel Characteristics and Morphology
Element 5. -- Fish Sampling
Element 6. -- Habitat Mapping

Element 1:
Database development by the BC Ministry of Water, Land and Air Pollution

Element 2:
Historical gravel removal.  Contact: M. Church

Along much of its length, Fraser River follows a steep, confined course through the mountains where it picks up sediment from the banks and from tributaries.  Within the Lower Mainland, the gradient of the river declines quickly as it approaches the sea and so gravel and sand are deposited between Hope and Mission.  These deposits are slowly aggrading – raising the river bed – as additional material is deposited there each year.

Aggrading rivers are laterally unstable.  They tend to shift their course frequently because significant deposits of sediment in the channel divert the flow, leading to bank erosion and lateral shifting of the channel.  How unstable a river is depends upon the size of the sediment load deposited annually in comparison with the size of the river.  Fraser River deposits are modest and the river is not highly unstable.

The consequence of gravel deposition and modest instability in the river is the development of bars, islands and secondary channels.  These features create aquatic and riparian habitat of exceptionally high quality because:

  • the reworked stable gravels provide spawning substrate for chum and pink salmon (up to two million in odd years);
  • the back channels and slackwater zones provide rearing habitat for several salmonid species, sturgeon and other fishes; and
  • the multiple channels provide extensive bankline where hiding zones and drop-in food sources occur.
The quality of the habitat depends not just upon the morphological complexity, but also upon continued change.  Gravel transport maintains a relatively loose streambed which the fish can work to excavate spawning redds, and it cleans fine material out of the gravels, allowing water circulation through the egg nests.

Gravel removal from the channel may disrupt this cycle of habitat renewal since the amount of “new” gravel entering the reach each year is small.  A key question is whether a level of gravel removal that would maintain flood safety is compatible with the maintenance of aquatic habitat.
 

Element 3: 
Sediment budget modelling. Contact: M. Church or Darren Ham, PhD candidate, UBC

Using bathymetric surveys between 1952 and 1999, a gravel budget has been established for the reach between Mission and the Agassiz-Rosedale Bridge.  A comparison of the surveys indicates that on average, approximately 270 000 m3 of gravel is transported past the bridge each year.  Experience from other rivers has shown that where significant volumes (amounts that exceed replacement rates) of gravel have been removed, channel morphology is simplified and biological diversity is reduced.

This does not imply that a rate of gravel removal that is restricted to equal the input of gravel will not impact fish habitat.  For the period 1964-1998, an average of 130 000 m3/yr of gravel has been removed from the reach.  This total is based on a recent UBC inventory, and probably represents a minimum estimate since some removals are undocumented.  Although these removals have not resulted in a significant change in the overall morphology of the reach, little is known about the time scale for ecological changes.

In addition to establishing gravel input rates, the surveys permit aggradation to be calculated along the river, revealing bed elevation changes between +1 metre and –1 metre over a period of years. Changes at this scale can substantially affect the margin of protection from flooding provided by the dykes.  Therefore, identifying active sites of deposition is an important step in managing flood protection.  A new survey of the river was completed this past summer.  We are currently updating the sediment budget to identify present sites of concern.

Element 4:
Channel characteristics and morphology. Contact: Darren Ham, PhD candidate, UBC

Wandering gravel-bed channels are common through mountainous and foothill regions of Western Canada. The term 'wandering' is used to describe a type of alluvial channel which exhibits similarities to both braided and anastomosed systems. Wandering rivers commonly form part of a downstrean continuum of channel planform types between meandering and braided reaches in response to varying environmental controls. Lower Fraser River flows in an irregularly sinuous single thread channel that is frequently split around large wooded islands, even at peak flows. Channel morphology is dominated by the erosion and mobilization of stored floodplain sediments. Gravel accumulates in locally unstable sedimentation zones. An extensive network of seasonal, perennial and abandoned sidechannels are typically found along the floodplain. These pathways delineate positions where the active channel flowed in the past, and are subject to reoccupation cutoffs during high flows.

These natural patterns of floodplain modification and channel instability create significant concerns for infrastructure developments, silvicultural investments, aquatic habitat diversity, navigation and recreation. These concerns are of particular significance along the lower Fraser River where applied management strategies (i.e. dyking, gravel removals) may conflict with perhaps the most important issue of all, the maintenance of aquatic habitat for the spawning, rearing and migration of the great Pacific salmon runs. Mitigation of potential environmental, economic and engineering conflicts, therefore, has practical implications for successful landuse and riverine management ofn the Fraser and similar mountain-valley rivers throughout British Columbia where similar concerns are shared or expected.

Floodplain modification and channel instability are directly related to the downstream transfer and storage of coarse alluvial sediments. Therefore, the key to improving our understanding is to examine the links between channel and floodplain morphodynamics and the contemporary sediment movement along these channels. This knowledge will also provide better tools with which to facilitate management decisions and mitigate resource conflicts. The study examines these links along the roughly 65 km long gravel-bedded reach of Fraser River between Mission and Laidlaw. The most suitable approach is to observe and measure patterns and rates of sediment transfer and morphologic development by measuring channel and floodplain changes at appropriate temporal scales, which is years to decades or more on lower Fraser River.

Element 5: 
Ecosystem study in the lower Fraser River. Contact: Laura Rempel, PhD candidate, UBC

Due to the logistical difficulties of working in large systems, the ecology of large gravel-bed rivers remains virtually uninvestigated.  We are conducting a 3-year fish habitat study in the gravel reach.  An important purpose of the study is to determine seasonal changes in habitat use and residency by various species.  Twelve sites have been selected for study, of which six have a history of gravel extraction.  Fish sampling at the twelve sites occurs four times per year to coincide with the stages of the flow hydrograph:

  • February – winter low flow;
  • April – spring rising water level;
  • July/August – summer high flow; and
  • September – autumn declining water level.
Over 45,000 fish have been measured and weighed to date.  Twenty-four species of fish have been identified, including 10 salmonid species, white sturgeon (red listed in BC) and 4 blue listed species (mountain sucker, coastal cutthroat trout, bull trout and Dolly Varden).  In an effort to link fish populations with habitat use, the following information is also gathered:
  • stomach content analysis of fish to determine the dietary selection of the different species;
  • sampling of aquatic insects on the channel bed in shallow (<40 cm) and of insects drifting in the water;
  • flow velocity;
  • water depth; and
  • substrate composition

We are sampling fish in a range of habitat types that differ in their physical character. These habitat commonly occur around bars throughout the gravel reach and appear to be ecologically distinct as well. Different fish species occupy various habitat types depending on the flow velocity, substrate and water depth. With this information, we are able to assess various sites along the river in terms of the abundance and diversity of fish that they may support based on the habitat types available at a given site. With information from our insect sampling, we are able to assess fish food production at various sites within the gravel reach and the importance of various insect types to fish as food.


Element 6:
Habitat mapping in the lower Fraser River. Contact: M. Church or Laura Rempel, PhD candidate, UBC

While it is recognized that gravel removal is potentially damaging to fish and to fish habitat, there is no clear understanding of the ecological impacts of this practice.  To date, fish and macroinvertebrate data collected from previously mined sites do not appear to show dramatic differences from those of unmined sites.  However, it is possible that the most significant impacts of gravel removal are short-lived (< 2 years).  To investigate this possibility, an experimental gravel removal was conducted for Harrison Bar toward the end of February, 2000.  Harrison Bar was chosen, in part, because our sediment budget data indicated that it is a site of recent major gravel deposition (and therefore a potential flood hazard site).

Several monitoring activities have been underway at Harrison Bar both prior to and since the gravel removal. We are monitoring fish use of habitats around the bar, aquatic insect production, and the physical recovery of the site in terms of surface substrate and gravel deposition. Monitoring activities are also underway at "reference" sites that are unmined. Our results remain preliminary at present, however, aquatic insects are found on Harrison Bar consistently since the gravel removal. Fish densities aroung the removal area fall within the range expected for an unmined site and a variety of habitat types are available for fish use. As well, the surface sediment on Harrison Bar was similar in texture to the unmined surface following the freshet in spring/summer 2000.

Fish appear to target particular habitat types for residency in the gravel reach, which differ in their physical conditions. We are linking the physical characteristics of the gravel reach with ecological attributes to produce a hierarchical classification of habitats. The classification consists of three levels, each level having a particular applicability to planning and management. At the highest level, we divide the river into 5 reaches that are distinguished on the basis of channel gradient, surface grain size, gravel transport rate, and pattern of aggradation. Taking note of these distinctions is believed to be important in strategic planning for fisheries management.

At the intermediate level, we identify pool-riffle units. There are 31 units in the gravel reach, averaging 2.6 km in length. These units correspond with the characteristic step-length for gravel displacement and are the largest identifiable units within which the full range of local habitat types may be found. Pool-riffle units are suitable for operational management and for planning of scientific studies and monitoring.

Habitat types around individual bars are the finest level of classification. Field surveys are required to identify these units with accuracy. There are twelve habitat types, which are both physically and ecologically distinct, and a typical pool-riffle unit may consist of 30-50 habitat units. Because water levels change dramatically through the year, the locations of habitat units change as well. We are continuing to refine the classification with field work and to determine if our method classifies habitat units consistently and therefore can be applied along the length of the gravel reach by air photo interpretation.

 


For information about the Fraser River Study, please contact any of the following:
Prof. Michael Church UBC   mchurch@geog.ubc.ca  (604) 822-6959 Darren Ham  Ph.D. candidate  fraser@geog.ubc.ca 

Funding for this project is provided by the BC Ministry of Water, Land and Air Protection, Canada Department of Fisheries and Oceans, City of Chilliwack, the Habitat Conservation Trust Fund of British Columbia, the Provincial Flood Assistance Program, Steelhead Aggregates Ltd., Indian and Northern Affairs Canada, District of Kent, the City of Abbotsford, and the Natural Science and Engineering Research Council of Canada (NSERC).