From the second edition (1989):
(snaɪ) Also snie, sny. [ad. Canad. Fr. chenail, Fr. chenal channel n.1]

A side-channel, esp. one creating an island.

1819 W. Keyes Diary 17 Apr. in Wisconsin Mag. Hist. (1920) III. 457 Evening, anchor a little above the upper snie (or channel) that leads to the Mississippi. 1826 Kingston (Ontario) Chron. 3 Nov. 2/5 We are also busy forming a channel through the rapids, for the sake of the raftsmen—this is done by building two strong dams, and deepening what is called a dry snie. 1829 J. MacTaggart Three Years in Canada I. 136 At this place, there are numbers of islands formed by snies winding round the Falls. 1886 in Alberta Hist. Rev. (1971) Summer 16/2 And from there to the snye which is a short cut into Fort Resolution. 1893 ‘Mark Twain’ in St. Nicholas Nov. 24/2 Ef we‥slips acrost de river to-night arter de moon's gone down, en kills dat sick fam'ly dat's over on the Sny. 1908 C. Mair Through Mackenzie Basin 40 Much of [the tracking]‥is in the water, wading up ‘snies’, or tortuous shallow channels‥floundering in gumbo slides. 1921 Beaver Aug.–Sept. 15/1 The Imperial Oil Company narrowly escaped the loss of their machines, which were lying on the snye at the back of the Fort awaiting favorable weather. 1948 Canad. Geogr. Jrnl. Mar. 150/2 The word snye, sny or snie has been used for many years to describe a channel behind an island, with slack current or partly dried, or some such similar feature. 1967 E. B. Nickerson Kayaks to Arctic ii. 17 There is a snye for float planes. 1969 E. W. Morse Fur Trade Canoe Routes ii. v. 57 The brigades shot the Allumette Rapids in their main (north) channel, the ‘Timber Snye’, where a safe canoe course passes.