. The Canadian field-naturalist. lough % Ii4«lrt L«lta. Figure 1. The three sections of the Trans-Alaska oil pipeline studied in winter 1977-78. the early 1960s, this herd has characteristically moved westward to its calving grounds in the Talkeetna Mountains between March and May. The herd returns to wintering areas, largely along the Gakona, Chistochina and upper Copper rivers, between October and January (Skoog 1968; Hemming 1971; Pitcher, Kenneth W. 1982. Susitna Hydroelectric Pro- ject, Phase n Progress Report, Big Game Studies, Volume IV. — Caribou. Alaska Department of Fish and Game. 43
. The Canadian field-naturalist. lough % Ii4«lrt L«lta. Figure 1. The three sections of the Trans-Alaska oil pipeline studied in winter 1977-78. the early 1960s, this herd has characteristically moved westward to its calving grounds in the Talkeetna Mountains between March and May. The herd returns to wintering areas, largely along the Gakona, Chistochina and upper Copper rivers, between October and January (Skoog 1968; Hemming 1971; Pitcher, Kenneth W. 1982. Susitna Hydroelectric Pro- ject, Phase n Progress Report, Big Game Studies, Volume IV. — Caribou. Alaska Department of Fish and Game. 43 pp.). In most years, these movements require crossing the Richardson Highway and since 1977, the pipeline as well. A Moose or Caribou crossing above-ground seg- ments of the pipeline does so through a rectangular "window". For the animal, this window is defined laterally by the vertical support members (VSMs) on either end of that pipeline segment, ventrally by the top of the pipeline pad, and dorsally by the bottom of the pipe (Figure 2). This rectangular window is 60 feet [ m] wide and of variable height. A single value was calculated to represent the vertical clearance (height) for each such window in the study area. This calculated value was the average of the clearances heights at the VSMs on each end of each 60-foot long segment. These VSM heights (distance from the top of the pipeline pad to the bottom of the pipe) were obtained from a list published by the Alyeska Pipeline Service Company (Listing of As-redesigned VSM Construction Section 1 and 2: 20 April 1977). The frequency distribution of calculcated vertical clearances was compared with the frequency distribu- tion of measured vertical clearances (top of pad to bottom of pipe) where Moose or Caribou tracks encountered elevated segments of the pipe. For these analyses, the data on calculated and measured vertical clearances were grouped in 1 foot [ m] intervals. However, because of their rarity
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