Between 400 and 600 year B.P., the Zigzag eruptive period produced deposits along the middle reaches of the Zigzag River and the upper Sandy River. Bouldery lahars and fluvial deposits created a terrace 8 to 10 m above present river level along the Zigzag and veneered a Timberline-age terrace on the Sandy River. The Old Maid eruptive period occurred between 180 and 270 years B.P. A single lahar flowed down the Sandy River at least as far as Brightwood, 30 km from Crater Rock. Sand deposits that may be related to this event are found down to the mouth of the Sandy River. A single lahar also flowed at least 18 km down the Zigzag River. The lahars, which were probably about 9 m deep, buried mature cedar forests along both rivers.
Since the end of the Fraser alpine glaciation about 10,000 years ago, three significant lahar-producing eruptive periods have occurred at Mount Hood. The lahars were confined to river basins originating on the southwest flank of the mountain. The older, timberline eruptive period occurred 1400 to 1800 year B.P. (Crandell, 1980). Deposits form the Timberline eruptive period (informally termed Timberline-age in this report) can be traced the length of the ZigZag and Sandy Rivers to the Columbia River. Flows from the Zigzag eruptive period (informally termed Zigzag-age in this report) have been identified only in the middle Zigzag basin and in the upper Sandy basin at Old Maids Flat. Deposits from the Old Maid eruptive period (informally termed Old Maid-age in this report) have been identified from the upper Sandy basin downstream to 2.5 km below the confluence of the Sandy and Salmon Rivers near the hamlet of Brightwood and at a single locality below the confluence of the Zigzag and Little Zigzag Rivers.
Although the lahars from the three eruptive periods affected all drainages on the southwest flank of Mount Hood, this paper will be limited to a discussion of the deposits in the Sandy and Zigzag drainage basins. These basins were chosen due to the concentration of population and recent intense development, which would exacerbate the effects of future eruptions.
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