LANDSCAPE AND VEGETATION ADJUSTMENTS IN ALAGANIK SLOUGH, ALASKA FOLLOWING THE 1964 GOOD FRIDAY EARTHQUAKE

Alaganik Slough is a river with a relatively small drainage basin with Saddlebag Glacier in the headwaters and the lower reaches merge with the large and complex depositional systems of the Copper River delta. Thus the headwaters are on bedrock (and till), but the lower parts of the river are essentially at sea level and therefore affected by tidal changes. The lower parts of the drainage area is characterized by disorganized drainage due to uplift and subsequent incision following the 1964 earthquake. Co-seismic uplift caused the area to uplift approximately 2 m and this uplift had different effects on river geomorphology and vegetation patterns above and below maximum tidal influence.

1) The lower reaches near the tidal limit (tidally influenced), which had been dominated by swampy areas with sedge and grasses in a low-relief setting, incised following uplift, and this incision allowed new growth of Picea sitchensis (Sitka Spruce) in drained areas adjacent to channels. Tree-ring chronologies of trees along the major drainage channel in the slough were established in 1980-1985 according to sampled groves.

2) The upper reaches of the river (above tidal influence) have incised downward and this incision has affected old (pre-earthquake) bank-lining trees through downcutting and bank erosion. Locally this incision is impeded by a thick and prominent peat layer with upright tree stumps from the penultimate megathurst event at ~913-808 Cal BP (Carver and Plafker, 2008). The modern bank-lining trees have undergone significant stress due to reworking of existing channels and bank failure that is likely driven by incision. Reaction wood in tilted trees (also P. sitchensis) indicate that incision-driven bank failure occurred in the mid 1970s, which is likely indicative of continued and progressive incision.

This study demonstrates that the growth histories of trees can be useful in understanding subtle changes caused by ground movement. This method could serve as a new approach to evaluating smaller-scale landscape changes associated with inter and co-seismic movement along subduction zones.

Carver, G., Plafker, G. 2008. Geoph. Mon. Ser. 179