British Columbia flood scars: maximum flood-stage indicators

Abstract

Flood scars, abundant along rivers of heavily forested northern British Columbia, provide records of flood occurrence and magnitude in a region with few and relatively short gaging records. Log transport during floods is episodic and occurs almost immediately preceding and during peak stage. Impact wounds form during this transport stage. Many of these logs are sequestered in bankside vegetation where they form abrasion wounds on trees by rubbing up and down in the turbulent waters. From May 30 through June 2, 1990, an approximate 9-year nival flood occurred on the Skeena River at Cedarvale, B.C. This flood damaged many trees within a few tree spacings of the bank margin. Immediately after the flood, the peak-stage line along 170 m of shoreline was marked. In September, 1992, 48 flood-scarred trees were selected and their locations and flood-scar heights surveyed. The mean height of the top of the flood scars sampled is 20 ± 3 cm below the peak stage on the adjacent bank. Flood-scar heights range from + 9 to − 80 cm compared to the peak stage on the adjacent bank. A regression line for the floodscar top elevations is roughly parallel to the peak-stage elevations along the shoreline and approximately 20 cm below it. Data from Usk, the nearest gaging station on the Skeena River, demonstrate that the stage was high enough to form the flood scars for only 24 to 36 hours. Most of the flood scars had to form in less than 24 hours. Field observations suggest that the time of flood-scar formation was even more restricted.