Alberta's boreal mixedwood forest has seen intensifying industrial activity in the past several decades, largely from logging and petroleum extraction. At the same time, populations of North American beaver (Castor canadensis) have been recovering from past near-extirpation. We conducted detailed field surveys of six beaver dam sites on low-order streams in northeastern Alberta and examined a 50-y chronosequence of air photos at each site to quantify beavers' effects on riparian forests. Beaver activity increased the width and diversity of riparian zones along first- and second-order streams. Over the 50-y time sequence, dam number increased considerably and beaver activity converted narrow, entirely lotic habitats to a mix of lentic and lotic. Current forestry operating ground rules in Alberta require 30- to 60-m unharvested buffer strips on permanent streams. Around dams, beaver felling removed most or all Populus trees within 30–40 m of the pond edge. The abundance of dams and their tendency to be built in chains altered vegetation structure over long stretches of riparian corridors. Beavers thus could be removing forest cover from entire buffer strips in direct conflict with forest management objectives. We argue that beavers may be the primary disturbance agent structuring riparian zones on low-order streams in the study area and that unharvested riparian buffer strips should be much wider than currently prescribed in order both to provide beaver habitat and to ensure appropriate protection of riparian habitats.