The Saskatchewan River basin yields perhaps the best available water and sediment discharge database for any major Canadian river basin (Ashmore and Day, 1988). In the reach around Outlook, Saskatchewan, the South Saskatchewan River flows in a valley about 0.6 km wide, with an average slope of 0.0003. Barfull discharge (equivalent to the elevation of Cant and Walker’s (1978) sand flats) is about 280 m3s-1, while bankfull discharge (to inundate islands and floodplains) is around 1240 m3s-1. The total drainage area is approximately 158,600 km2, although the contributing area is only around 85,700 km2, since the rolling and hummocky topography of the glacially sculpted Prairies cause large areas to be internally drained (Ashmore and Day, 1988).
30 km upstream of Outlook lies the 64 m-high, 4.8 km-wide Gardiner dam, which impounds about 9.4 x 109 m3 of water to form Lake Diefenbaker (Rasid, 1979). Construction of Gardiner Dam commenced in 1962 and was completed in 1967. In terms of the hydrological regime, the closure and operation of Gardiner dam resulted in a decrease in the magnitude of high flows, an increase in the magnitude of low flows, and a re-distribution of high flows from the spring and early summer, representing snowmelt runoff, to the winter months. Suspended sediment concentrations have been substantially reduced (Rasid, 1979), with mean concentrations falling from 350 to 30 mg l-1 and bankfull concentrations falling from 1400 to 650 mg l-1. However, early post-dam research (Rasid, 1979) noted bed lowering over the study reach of only 0.2 m, a figure Cant and Walker (1978) found to be “much less than local fluctuations due to channel shifting” (p.627). More recent research (Phillips, 2003) suggested that there had been minimal impact on the mean bed elevation from around 13 km downstream of the dam. In addition, Cant and Walker (1978) noted that a comparison of aerial photographs obtained before and after the emplacement of the dam shows that the planform of the river, in terms of its braid-bar scale, type and activity, is unchanged. A crude aerial photo mosaic demonstrating the range of bedform scales evident in the river can be seen in Figure 1. With close inspection, one can see forms ranging from dunes to unit bars to complex bars and bar complexes.
Figure 1. Aerial photo mosaic of the South Saskatchewan River near Outlook, Saskatchewan. Click on the image to download the mosaic in (zipped) MrSID format. Along with various GIS and CAD software, MrSID files can be displayed with either a free viewer or a Windows® only web browser plug-in . Both are available from LizardTech, Inc. This format allows the user to zoom in on the image while simultaneously increasing image resolution so that definition is not lost. The images have been converted from 10 µm resolution to ~21 µm resolution to allow more convenient download.
Given the lack of a systematic change in braiding dynamics, this suggests the limited role of silt- and finer-sized sediment in the braiding mechanism, at least for Prairie and Plains Rivers (see also Brice, 1964).
Figure 2. Aerial photograph of the North Loup River near St. Paul, Nebraska modified from Brice's (1964) figure 17.
Figure 2 shows an aerial photograph of the North Loup River, Nebraska, taken from Brice's figure 17; it is difficult not to draw visual comparisons with the scale and form of the braid bars in Figure 1. Brice's research also emphasised the fact that these rivers derive most of their total load from local bank erosion and/or overland flow eroding surficial material. Indeed, Ashmore and Day (1988) noted that for the South Saskatchewan the glacial deposits and erodible Cretaceous sediments permit mass movements, extensive gullying and the development of badlands, promoting the dominance of local riparian erosion as a sediment source. Brice found geological evidence to indicate that the Loup Rivers were slowly degrading, suggesting that for these rivers degradation is not an unnatural phenomenon.
Ashmore, P.E. and Day, T.J., 1988. Spatial and temporal patterns of suspended-sediment yield in the Saskatchewan River basin. Canadian Journal of Earth Sciences 25 (9), 1450-1463.
Brice, J.C., 1964. Channel patterns and terraces of the Loup Rivers in Nebraska. USGS Professional Paper 422-D. Washington: US Government Printing Office.
Cant, D.J. and Walker, R.G., 1978. Fluvial processes and facies sequences in the sandy braided South Saskatchewan River, Canada. Sedimentology 25, 625-648.
Phillips, R.T.J., 2003. Downstream geomorphic impacts of reservoir construction on the sand-bed braided South Saskatchewan River, Saskatchewan. BSc (Hons.) Dissertation, Department of Geography, Simon Fraser University, Burnaby, British Columbia.
Rasid, H., 1979. The effects of regime regulation by the Gardiner Dam on downstream geomorphic processes in the South Saskatchewan River. Canadian Geographer 23 (2), 140-158.