Previous research shows an anomalous absence of plains cottonwood (Populus deltoides) woodlands along a 30 km sandy braid-channel reach of the Milk River upstream of the Fresno Dam in northern Montana, USA. This absence contrasts to well-wooded, meandering reaches upriver and downriver of the braid-plain. Our field measurements show that mechanical ice breakups and ice floes physically damage most plains cottonwood saplings and small trees along the braid-plain. Between 1911 and 1998, mechanical ice breakups occurred in 50% of the years of record. Of these, 12% were greater than bankfull discharge and followed dramatic temperature increases and subsequent snowpack melting in the headwaters. We suggest that the absence of cottonwood woodlands on the braid-plain is associated with low bank heights, wide channels, and low channel sinuosity in the braiding reach through which overbank river ice moves more easily compared to the meandering reaches. Ice blocks rafted onto the braid-plain have repeatedly scarred, bent, broken (trunks partially detached, roots not displaced), sheared (trunk detached, roots partially displaced), and toppled (trunks and roots displaced) cottonwoods, causing an absence of middle- to old-aged trees. Even though hundreds of new stem and stump shoots have sprouted since the last major ice damage (1996), very few trees are present on the braid-plain compared to the meandering reaches. Almost all (94–100%) cottonwoods on the braid-plain were damaged by ice in 1996 (but only 23% in the meandering reaches). All damaged cottonwoods were scarred; up to 42% of the ice-damaged trees on the braid-plain died from shearing and toppling. Wetlands in the U.S. and Canada are defined on the basis of frequency of flooding and presence of water-logged soils that support hydrophytic plants. Ice plays a critical role in limiting plant distribution in northern riparian wetlands and should be incorporated into revisions of existing classification systems.
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Vol. 20 • No. 2