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Effects of river ice on bank morphology and riparian vegetation : Peace River, Alberta Uunila, Lars Sakari

Abstract

This study investigated the effects of river ice and related flooding on the bank morphology and riparian vegetation along 655 km of the Peace River from Clayhurst, British Columbia to Fort Vermilion, Alberta. Regulation of this river since 1968 for the generation of hydroelectric power has caused a change in the flood regime and has resulted in variable channel adjustments and patterns of riparian succession. A major objective of this study was to determine how bank morphology and riparian vegetation might be influenced by disturbances created by ice jams and ice runs. Historical observations of ice jams have been focussed near the Town of Peace River, the largest population centre along the river. In order to examine frequency and magnitude of ice jam events for the remaining, sparsely populated reaches, a river-length reconnaissance was carried out. Both the direct physical effects of ice and the indirect effects of ice jam flooding on the channel margin were investigated. Bank morphology, sedimentary features, and vegetation patterns were surveyed on 270 transects of the riverbank at locations chosen to encompass as wide a range of hydrogeomorphic conditions as possible. During each transect, the extent of vegetation damage in the form of broken stems and scars that could be attributed to ice jams, ice runs, or related flooding was measured. Approximately 300 wedges and cross-sectional samples of ice-damaged woody vegetation were collected and analyzed to determine the date of past ice disturbances. Analyses of the geomorphic and botanical evidence collected in this study suggest that the frequency and magnitude of high stages due to ice jams peak near the central part of the study reach, between the Town of Peace River and Carcajou. This reach is highly confined and sinuous with a large number of mid-channel islands, confirming that channel morphology is a major factor determining ice jam location. Ice shove levels within the study reach peak at an elevation of 11 m above the mean summer stage. Based on the tree scar record along the river, ice jam flooding has occurred in 27% to 86% of the post-regulation years. Frequent events occur along a confined sinuous channel with a significant gradient reduction near Notikewin River. Ice jams occur less frequently in the upper Peace River, where the formation of an ice cover is intermittent. Ice scour and ice jam floods appear to maintain the active shelf, a characteristic bench found between the pre-regulation floodplain and the current, post-regulation channel bed. Although both erosional and depositional processes operate on this bench, evidence suggests the sedimentation during ice jam floods is most important. Flooding and sedimentation from ice jams also affect secondary channels that have rarely been inundated during open-water floods following regulation. Scour and push features are irifrequently scattered along the study reach and are short-lived due to the inherent erodibility of the sediments. The most conspicuous effects of ice jams and ice scour are found in the riparian vegetation. Numerous bent, broken and scarred stems caused by ice suggests that ice disturbance is frequent along the lower seres, particularly in high exposure areas such as island heads and cut banks. However, for the same elevations above the river, vegetation tends to be older and less disturbed in the lower reaches than the upper and middle reaches, due to differences in channel morphology that cause ice jams to halt along wide bar and bench surfaces.

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