River ice and its role in limiting woodland development on a sandy braid-plain, Milk River, Montana

Abstract

Abstract 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 repeatedl...