Photo by Phyllis C. Adams 1992 . With a length of nearly 1000 km, the Tanana River is the largest tributary of the Yukon River. During the Delta Glaciation of the late Pleistocene, the increased discharge and sediment load from the glaciers of the Alaska Range caused the Tanana River and its tributaries to aggrade rapidly, damming the lower reaches of several valleys of the Yukon-Tanana Uplands. A period of downcutting by the Tanana followed the end of the Delta Glaciation, forming an upper terrace. The recent geological history of the Tanana River is only partially documented.
Sediment input is related, ultimately, to production in glacial sources in the Alaska Range.
Such changes in sedimentation rates presumably reflect changes in the frequency of floods large enouth to overtop terraces. Similar ages were found by Mann et al. Both are correlative with glacial expansions. The intervening period from 2300 to 400 yrs ago of comparable surface stability on the floodplain recorded frequent fires and soil formation on the terrace which reflects warmer and dryer conditions. Preliminary mapping in the BCEF area of the floodplain confirms this basic chronology and shows that the islands contain floodplain deposits 1000 yrs old.