Abstract. The first direct measurements of evaporation from a large high-latitude lake, Great Slave Lake, Northwest Territories, Canada, were made using eddy covariance between July 24 and September 10, 1997, and June 22 and September 26, 1998. The main body of the lake was ice-free between June 20 and December 13, 1997, and June 1, 1998, and January 8, 1999, with the extended ice-free season in 1997-1998 coinciding with 4øC above normal air temperatures and an abnormally strong E1 Nifio. Measurements extending roughly 5.0 to 8.5 km across the lake were made from a small rock outcrop located near the main body of the lake. The lake was thermally stratified between midJuly and September, with the thermocline extending down to approximately 15 m. High winds were effective in mixing warm surface waters downward and, when accompanied by cold fronts, resulted in large, episodic evaporation events typically lasting 45 hours. The daily total evaporation was best described as a function of the product of the horizontal wind speed and vapor pressure difference between the water surface and atmosphere. Seasonally, the latent heat flux was initially negative (directed toward the surface) followed by a steady increase to positive values (directed away from the surface) shortly after ice breakup. The latent heat flux then remained positive for the remainder of the ice-free period, decreasing midsummer and then steadily increasing until freeze-up. The sensible heat flux was small and often negative most of the spring and summer yet switched to positive and began to increase in the early fall. Extrapolation of evaporation measurements for the entire ice-free periods gave totals of 386 and 485 mm in 1997 and 1998-1999, respectively.

ABSTRACT. Hydrometric and sediment data collected by Environment Canada in the Mackenzie Basin during the period 1974-94 have been analyzed to produce detailed estimates of sediment inputs to the Mackenzie Delta, based largely on sediment rating equations. The mean annual sediment supply to the delta is determined as 128 million tonnes (Mt), of which about 4 Mt is sandy bed material moved in by the Mackenzie River itself. Virtually all of this sediment (more than 99%) is supplied to the delta during the May-October period, the peak months being May (27%), June (36%), and July (19%). About 17% of the fine-sediment load is supplied by the Peel River; the rest is delivered by the Mackenzie. The largest single contributor to the Mackenzie River wash load (103 Mt) is the Liard River (41 Mt). The preliminary estimate of the contribution of the other west-bank tributaries, in combination, is about 36 Mt, though this figure is probably too low. The precision of these estimates using the sediment rating approach (compared to time-integration during months with reasonable sampling frequency) is about 10% for the mean monthly sediment loads and about 5% for the mean annual sediment load during the 1974-94 period. The absolute accuracy of sediment load estimates is more difficult to assess because published flow data for delta inflow stations are acknowledged to be much less reliable for the spring breakup period than for other times of the year.

Abstract:Data acquisition by Environment Canada and others over the last 20 years now allows the ®rst comprehensive synthesis of the riverine sediment balance of the Mackenzie Delta. The data presented here are: sediment inputs from the Mackenzie and Peel rivers at the delta head and river sediment transfers from the Upper Delta to the Outer Delta (1974±1994); in-channel and overbank sedimentation, including lakes ( post-1963); and in-channel erosion along Delta channels (1950±1981). These data indicate that the mean annual sediment input to the Delta is about 128 Mt, and the corresponding loss to oshore is about 85 Mt. The net sedimentation of 43 Mt is divided almost equally between the Upper Delta (mostly on levees and lake beds) and the Outer Delta (mostly on lake shores). Gross sedimentation within the Delta, about 50% of which is on point bars, is much higher, estimated at about 103 Mt annually: the dierence is the large amount of sediment reentrainment within the Delta, through bank erosion, primarily along Middle Channel. How much of this pointbar deposition is from settling of sediment delivered by the Mackenzie and Peel rivers (as distinct from local sediment derived from bank scour within the Delta) is not known. Such within-Delta sediment exchange (which could be as high as 50 Mt) might be important in determining the quality of sediment (nutrients, contaminants, etc) that it is being delivered oshore: it would be naõÈ ve to assume that all of this sediment is from the present-day input of the Mackenzie and Peel rivers.