. The Canadian field-naturalist. 426 The Canadian Field-Naturalist Vol. 118. losow 100°W Figure L Location of study area (A), including the Horton River (B) and Thelon River (C) showing sample sites. The fine dashed line indicates the watershed boundary upstream of the lowest sampled site for both watersheds. The grey border on the Thelon Watershed map indicates the boundaries of the Thelon Game Sanctuary. Site co-ordinates are given in Table 1. a drainage basin area of approximately 26 680 km- and a total length of = 800 km, runs entirely in the Southern Arctic Ecozone. Expedition members col
. The Canadian field-naturalist. 426 The Canadian Field-Naturalist Vol. 118. losow 100°W Figure L Location of study area (A), including the Horton River (B) and Thelon River (C) showing sample sites. The fine dashed line indicates the watershed boundary upstream of the lowest sampled site for both watersheds. The grey border on the Thelon Watershed map indicates the boundaries of the Thelon Game Sanctuary. Site co-ordinates are given in Table 1. a drainage basin area of approximately 26 680 km- and a total length of = 800 km, runs entirely in the Southern Arctic Ecozone. Expedition members col- lected over a 700 km stretch of the river from Horton Lake to the Arctic Ocean. The river flows over lime- stone-dominated substrates in its upper reaches, where it is characterized by relatively high pH (>) and spe- cific conductance (>150 |iS/cm). In its lower reaches, the river flows through the Smoking Hills, a region containing lignite and magnesium deposits which spon- taneously ignite when exposed to air (for example, during bank slumping during summer). These result in "smoke holes" as smoke pours from places where the coal is burning underground, and the water drain- ing these sites is profoundly affected by the geology. Tributaries and ponds in this region were highly acidic (some with pH values < ), and acid runoff in this area resulted in a decline in river pH to approximately neutral values. The river is clear and swift-flowing for most of its length, but becomes muddy and meander- ing in its lower portion. Sampling was carried out in 37 sites on or near the river between 17 July and 8 Au- gust 2000. Predaceous water beetles were collected from 5 of the 37 sites along the river, corresponding to sampling sites 8, 11, 14, 18, and 23 of the larger Hor- ton River survey; all were located in the middle sec- tion, which was clear and swift-moving (Figures lb, 2; see Table 1 for descriptions of each site). The Thelon River begins near Lynx Lak
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