The Tanana River

A river of change

The Tanana River floodplain ecosystem is a major focus of the Bonanza Creek Experimental Forest (BCEF) LTER studies. Located in central Alaska (63�-65� N) about 150 to 250 km south of the Arctic Circle, the Tanana River flows northwest from headwaters near the Canadian Border. With a length of nearly 1000 km, the Tanana River is the largest tributary of the Yukon River. Bordered by the Alaska Range to the south and the Yukon-Tanana Upland physiographic province to the north (Wahrhaftig 1965), the Tanana River valley lowland covers 17,400 km2. The Tanana River occupies a structurally-controlled basin extending below sea level and is filled by Tertiary and Quaternary fluvial/glaciofluvial deposits as much as 100 to 250 m thick (Pewe and Reger 1983).

Geomorphic Development

Early geological history of the Tanana River was summarized by Collins (1990). 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. During the Donnelly Glaciation, the Tanana River once again aggraded; the resulting floodplain was not built up as high as during the Delta Glaciation. Following the end of the Donnelly Glaciation, the Tanana cut down again, forming a second, lower terrace, whose age has been estimated at 10,000 years before presetn (Blackwell 1965).

The recent geological history of the Tanana River is only partially documented. Fluctuating periods of minor aggradation and downcutting are probable; these periods possibly correspond to minor Holocene climatic changes or alpine glaciations (Collins 1990). Documentation of late Quaternary alluviation along the Tanana River is outlined by Mason and Beget (1991) . Illinoian(?) and Wisconsin-aged terraces and meander scars along the Tanana were described by Pewe et al. (1966). Fernald (1965) first suggested that Tanana River alluviation varied throughout the Holocene from studies of terraces in the upper Tanana basin. The Bitters Creek section near Northway recorded high sedimentation rates (1.7 m/1000 yr) during the middle Holocene between 6200+300 BP to 5380+260 BP and lower sedimentation rates after 1600 BP (less than 0.2 m/1000 yr). Stratified archaeological sites along Tanana River tributaries suggest high rates of deposition between 4000-2000 BP and before 1100 BP (Leehan, 1981). 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.

Active and recently abandoned floodplain terraces adjacent to the Bonanza Creek Experimental Forest are only around 3000 years old, based on a series of radiocarbon ages on stratigraphic sections in Luke's Slough (Mason and Beget 1991). Similar ages were found by Mann et al. (199The stratigraphic succession preserves a record of late Holocene alluvial flooding. Two periods witnessed considerably more frequent and larger floods, 3000-2300 yrs ago and from 1600 to 1900 AD. 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. However, most of the islands are considerably younger, less than 400 yrs old.

River Discharge and Floods

About 85% of the discharge of the Tanana River is derived from north-flowing, glacier-fed rivers originating in the Alaska Range. Four large rivers provide about half the total Tanana Discharge, and two of these, the Nabesna and the Delta, lie upstream from the study locale. The south-flowing tributaries from the Yukon-Tanana upland provide only about 15% of the total discharge and all lie upstream from the study locale (Anderson 1970). Baseline data for studies of environmental change along the Tanana River are available from historic records of river activity. Unfortunately the duration of gauged discharge record for the Tanana River and its tributaries is quite short, extending only over portions of the last forty years (Bigelow et al. 1989). On the Tanana River itself, gauging stations were maintained only at Tanacross (1953-present), Big Delta (1948-52, 1953-57), Tok (1950-53, and Nenana (1962 to present). Gauging stations have also been maintained sporadically at eight other locations on tributaries of the Tanana River, with the longest period of record at the Chena River station at Fairbanks (1948 to present).

Reported annual maximum discharge for the Tanana River shows a predictable downriver increase, with upstream discharge reocrded at 1107 m3 s-1 (39,100 cfs) near Tanacross building to 3310 m3/s (117,000 cfs) near Nenana in the middle reaches of the river. As is common in arctic rivers (Church 1988), the Tanana varies widely in mean monthly discharge, with minimum values occurring during the lengthy period of low discharge under the winter snow and ice cover and much higher discharges characterizing break-up periods in spring. Stream flow at Nenana in January falls to only 280 m3/s (10,000 cfs) while June discharges are often over 1415 m3/s (50,000 cfs). Ice jams and localized overbank floods are fairly common along tributaries and at some localities along the Tanana River during spring breakup and are associated with a rapid warming over several days (Smith 1978).

The most detailed flood records in the Tanana basin derive from Fairbanks, the largest and oldest settlement in the region. Flood records at Fairbanks refer primarily to the Chena River, a comparatively short south-flowing tributary of the Tanana. Flood stage has reached over 4.5 m above datum in Fairbanks at five times during the last 90 years: in May of 1905, 1911, and 1937, and in August of 1930 and 1967 (Bigelow et al. 1989) The May floods in 1905, 1911 and 1937 were caused by local ice jams (Bigelow et al. 1989, Pewe 1982). Much larger floods have been produced by unusual meteorological events. The 1967 event is the flood of record and peaked at a discharge of almost 10 times greater than the average summer maximum of 212 m3/s (7500 cfs). Downriver, at the Nenana gaging station on the Tanana River, the August 1967 flood had a discharge of 5260 m3/s (186,000 cfs) and a gauge height of 5.76 m. This flood was produced by a series of large rainstorms in August which affected much of the Tanana River basin (Childers et al. 1972). The Tanana and Chena Rivers both flooded and submerged all of Fairbanks and large areas of the floodplain along much of the length of the Tanana River, causing substantial damage along virtually the entire floodplain (Childers et al. 1967). In addition, high precipitation in the Tanana upland produced debris landslide in the Chena River area.

A flood due to rainstorms in May 1948 on the Chena and Tanana Rivers had a discharge at Nenana of 3820 m3/s (135,000 cfs) and a gauge height of 4.85 m, and an estimated discharge of about 680 m3/s (24,200 cfs) on the Chena River (Pewe 1982). Up river from Fairbanks on the Tanana River, small floods occurred at Tanacross during June 1962 and July 1975, elevating discharge about 25% over annual extreme values (Bigelow et al. 1989).

The frequency of large floods has been estimated from the relatively short gauge records of discharge. Collins (1990) used 15 yr of gauging data (1973-1987) from the Tanana River at Fairbanks to calculate a recurrence interval of 333 yr for the 1967 flood. Pewe (1982) suggested that floods as large as that in 1967 have a recurrence interval greater than a century, while the 1948 flood had a recurrence interval of 25 years. Mason and Beget (1991) estimated that large floods occurred every 57 years during the last 400 years; an increase over the timing of floods in the previous millennia that witnessed a large flood every 120 years.

Several generalizations can be made about Tanana valley flooding. While floods are caused by both ice dams during break-up and by high rainfall storms, floods following break-up are usually more restricted in area and size. By far the largest floods are produced by unusual meteorological conditions, i.e., very high rainfall over several days. The two types of flood differ in their ability to transform the landscape because of the occurrence of frozen ground (both annual and permafrost) which protects river banks from erosion during break-up floods. Maximum riverbank erosion in June or July, during periods of high water, is dependent on warmer water temperatures, as at Galena, on the Yukon River (Ashton and Bredthauer 1986).

The sediment yield accompanying particular types of flood also differs considerably. In upland areas around Fairbanks runoff is highest during break-up, April-May, when the ground is still largely frozen (Gieck and Kane 1986). This nival run-off pattern connected to the spring melting of the cumulative snowpack adds very little sediment to the Tanana River, and probably adds little to alluvial terraces. Similarly, flooding induced by ice jams is unlikely to contribute significantly to overbank sedimentation (Smith 1978:335). The catastrophic summer flood is more likely to erode banks, incorporate sediment into suspension and produce overbank deposits. Glacial streams also undergo an annual cycle of flooding connected with the warmth of the summer season which results in the transport of sediment downstream. Infrequent large-scale glacial floods occurring in summer probably contribute a considerable amount of sediment into the Tanana River and its tributaries.