The Mackenzie River regime

The rivers flowing into the Mackenzie Delta are the Mackenzie River, the Peel River, the Rat and the Rengleng Rivers (fig. 2.2). The Arctic Red River flows into the Mackenzie River (fig. 2.2). The comparison between the mean annual discharge of each one of these rivers (table 3.1) indicates that the Mackenzie River at Point Separation (fig. 2.2) contributes about 93% of the total input in the delta. The Peel river discharge represents about 7% of the total input while discharges in the Rat and Rengleng rivers are minor compared to the total inflow. Discharge data on the Big Fish river (fig. 2.2) are very sparse, but they show that discharges are very low and comparable with the Rat and Rengleng rivers (LEWIS, 1988).

Distribution of waters within the delta originating from the Mackenzie River or from the Peel River was estimated by MACKA Y (1963) by identifying ali drainage sub-basins within the delta. The contribution of the Mackenzie River is predominant in the whole delta except in the western part, in the following areas:

• the south-western part of the delta, between the mouth of the Peel River and Aklavik, receives mostly waters coming from the Peel River and from the Rat River; the contribution of the Mackenzie River in this area is estimated to be around 10%. Husky channel (fig. 2.2) is mainly fed by small mountain rivers;

• north of Aklavik, channels flowing to the south west of Shallow Bay (west of Napoiak Channel) con tain about half Peel River and mountain river water and half Mackenzie River water;

• the flow in the area lying between West Channel and Shallow Bay in the north-western part of the delta are estimated to contain approximately 85% Peel and river mountain water and 15%

Mackenzie water.

Flow distribution through the northem part of the delta is estimated to be about 20% through East Channel into Kugmallit Bay, 30% into Mackenzie Bay, 20% through Reindeer Channel and 30% into

-43-Chapter 3: Sedimentation in the Mackenzie Delta

Shallow Bay (ANDERSON and ANDERSON, 1974; HOLLINGSHEAD and RUNDQUIST, 1977; LEWIS, 1988).

Table 3.1: River inflow into the Mackenzie Delta; mean discharge are calculated on daily mean discharges provided by the Water Survey of Canada (WSC)

Hydrometrie station Mean annual RSD¹ of annual %of total Period of discharge [m³/s) mean discharge [%] input record

Mackenzie River above 9729 9.7 91.8 1973-1993

Arctic Red River (10LA003)

Peel River above Fort 701 12.6 6.6 1975-1993

McPherson (10MC002)

Arctic Red River near the 158 15.1 L.5 1974-1983

mou th ( 1 OLA002)²

Rat River near Fort 7.5 0.07 1983-1984

McPherson (10MC007/

Rengleng River below 3.1 33.7 0.03 1974-1984

Dempster Highway (10LC003)²

Total 10,598.6 100.0

1 RSD =relative standard deviation ² data of WSC presented by LEWIS (1988)

The year-to year variation in annual discharges of ri vers flowing to the Mackenzie Delta is expressed by the relative standard deviation (RSD). The RSD for the Mackenzie River is slightly lower than the

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RSD of the Peel River and the Arctic Red River. Year-to-year vanatwn of the Mackenzie River discharge is influenced by the irregular discharges of the Liard River (RSD around 17%); the influence of the Liard River is higher during spring period, as waters corning from the Liard River at Point Separation make up over 30% of the flow (LEWIS, 1988).

The profile illustrating the annual mean discharge as a function of time (fig. 3.4) displays an oscillatory component with two Figure 3.4: Annual mean discharge of the Mackenzie maximal values in 1975 and in 1988 and a River above Arctic Red River between 1973 and 1993 low discharge period between 1980 and

1975 1980 1985 1990

years

1983. Nevertheless, the period of record is too short to verify a potential discharge cycle. It should be noted that there is no relationship between flood and discharge because of ice resistivity within the delta. Indeed, according to LEWIS (1988), "the development and release of ice jams are the primary controls on both the progress of break-up and ice clearance and on the peak water levels which determine the extent, depth and duration of flooding of the delta plain surface".

Seasonal flow periods

Discharges of the Mackenzie River fluctuate throughout the year because of seasonal changes of climate within the delta and within the river system. The daily discharge profile (fig. 3.5) from January to December can be separated in three periods corresponding to characteristic seasonal thermal regimes:

• the winter freeze-up,

• the spring break-up,

• the summer open water.

The winter begins in late September when smalllakes within the delta start to freeze-up. A thin caver of ice begins to caver larger lakes, distributary channels and the neashore coastal zone in early October

(LEWIS, 1988). By late October to mid-November, the delta and the Beaufort Sea becomes entirely

Figure 3.5: Envelope of daily mean discharges of the Mackenzie River above Arctic Red River between 1973 and 1993 (data provided by the Water Survey of Canada). along shallow distributary channels (JENNER and HILL, 1991), the Mackenzie River discharge and suspended sediment concentration increase rapidly (fig. 3.5). The large volume of water becoming confined between river mouths and seaward bottornfast ice induces the flooding of delta front areas less than one meter in height (JENNER and HILL, 1991). The peak fluvial discharge occurs in early June or in late May. The large volume of water arriving at the delta front overflow the bottornfast ice, accelerating the melting process. Flow velocities can exceed 1 mis in Middle channel and 0.4 mis in smaller channels.

It should be noted that there is no relationship between flood magnitude and discharge as ice resistivity is the main factor controlling flood magnitude.

The open-water season starts in late June after the peak discharge and continues until October when the first sign of ice appears. During this period, the Mackenzie River discharge is controlled by

-45-Chapter 3: Sedimentation in the Mackenzie Delta

hydrometeorological conditions in upstream tributaries (BIGRAS, 1987) such as the Liard River and by storm surges affecting the delta front (MARSH and SCHMIDT, 1993). Peak flows during summer can exceed discharge maximums during the break-up as in 1988 or in 1974 (fig. 3.5), but these summer events are much shorter. Because of the absence of ice effects, water levels on the delta distributaries are lower than levels reached during the spring break-up. However, the increases of channel water levels are stiJl significant, especially for low sill elevation lakes, causing flow reversais in connected lakes.

Sediment inputs to the delta

Daily suspended sediment concentration (SSC) has been measured for severa! years from late May to early November in one station at the head of the delta, Mackenzie River above Arctic Red River (IOLC14). Intermittent measurements have been performed on the Peel River and on the Arctic Red River. It should be mentioned that bed Joad is not included in the SSC measurements. It was estimated that bed load accounts for less than 5% of the total sediment input to the delta (CARSON, 1994a).

Records of mean daily SSC Mackenzie River above Arctic Red River (data measured in 1973 and from 1980 to 1991 provided by the Water Survey of Canada)

the total amount of sediments passing the Mackenzie above Arctic Red River station, rising to over 55%

during May and June (B.C. HYDRO, 1984). At the coast, winds and waves become the dominant mechanisms for sediment transport and deposition in the nearshore zone (JENNER and HILL, 1991).

Fluctuations between years, from 1980 to 1991 are very high with the relative standard deviation of SSC around 67%, which is much higher than the RSD of discharge (20%).

Annual sediment load at the head of the delta can be estimated on the basis of SSC and discharge data measured at the three head stations (Mackenzie River above Arctic Red River, Peel River above Fort McPherson and Arctic Red River near the mouth). Using load estimates for the period 1974-1990, the total mean annual suspended sediment input to the delta was estimated by CARSON (1994a) as about 127 Mt of which the Peel River appears to be 22 Mt. LEWIS (1988) based on a shorter period of time (1974-1983) figured out a total annual input of 126 Mt to the delta including 30 Mt coming from the Peel River. The difference between the two estimates arises from the larger contribution of the Peel River in LEWIS' estimate. Other estimates on the total sediment input to the delta were performed (INMAN and NORDSTROM, 1971, LISITZIN, 1972; MILLIMAN and MEADE, 1983; HIRST et al., 1987), but they are not described here as they were based on very little concentration data and sometimes on extrapolations from

unmeasured basins (LEWIS, 1988).