Lake sedimentation in the Mackenzie Delta, NWT

Abstract

Spatial variation in lake sedimentation within the Mackenzie Delta was documented, and total lake sedimentation over the past 30 years was estimated, by classifying (via aerial photography) the flooding characteristics of a complete transect of lakes (about 2600 lakes) across the upper subaerial delta plain, then selecting representative samples of lakes (81 lakes) from the transect, and a random sample of lakes from a second transect (7 lakes) across the lower subaerial plain, for direct measurement of sedimentation rates. The depth corresponding to the 1963 sediment layer was obtained from a sediment core from each of these lakes by sampling the core at frequent depth intervals to identify the peak in 137Cs (Caesium-137) content. Lake sediment bulk densities, as obtained from the cores, averaged 931 kg/m3, a value lower than reported in previous studies. Using mean bulk density above each 137Cs peak, and the depth of the 137Cs peak, mean lake sedimentation for different lake types varied between 1.19 kg/m2/yr and 5.34 kg/m2/yr, with a decrease in sedimentation as lake sill elevation increased. Values for the lower delta transect averaged 1.36 kg/m2/yr. By combining relationships between sedimentation and lake type with a classification of lake types, total sedimentation across the upper-delta transect is estimated as 8.2 Mt/yr, and 0.8 Mt/yr for the lower delta. These values are lower than previously reported primarily because they are derived from lower bulk density values. Of the total upper-delta sedimentation, 17% occurs in high-closure lakes, 53% in low-closure lakes, and 30% in no-closure lakes. Lake sedimentation is higher in the Middle Channel segment of the upper-delta transect (44% of total sedimentation across the transect) than in the East Channel (25%) or Peel Channel (32%) segments.