1. Transport and Fate of Spring Floodwater from Rivers in the Alaskan Beaufort Sea
John H. Trefry1, Robert P. Trocine1, Carrie M. Semmler2, Matthew B. Alkire3, Mark A. Savoie4 and Robert D. Rember5
1Florida Institute of Technology, Melbourne, FL 32901, 2Louisiana Universities Marine Consortium, Chauvin, LA 70344, 3Oregon State University, Corvallis, OR  97330, 4Kinnetic Laboratories, Inc., Anchorage, AK 99501, 5IARC, University of Alaska Fairbanks, Fairbanks, AK 99775
1. Introduction
Most rivers that drain into the Arctic Ocean carry 40-80% of their annual volume of water, suspended solids and dissolved chemicals during the 2-3 week period of the spring floods. In many cases, these large seasonal discharges are carried to an ocean covered with ~2-m thick ice (Figure 1). Much of the spring flow is carried offshore under the ice without mixing by winds and waves. The magnitude, timing and fate of riverine inputs in the coastal ocean have important consequences on the hydrology of the Arctic and on estuarine food webs. Observed variations in river composition, flow and mixing with coastal seawater during this multi-year study provide insights to and an opportunity for discussion of possible responses to environmental change in the Arctic
3. Riverine Inputs (continued)
Concentrations of total suspended solids (TSS) in the Sagavanirktok and Kuparuk rivers typically peaked during the first week of the spring floods (Figure 5). Peak values for TSS in the Sagavanirktok River ranged from 249 mg/L in 2002 to 609 mg/L in 2001 and were much higher than the range of peak values of mg/L for the Kuparuk River that has no mountain source of suspended particles. Water and sediment discharge from the Kuparuk River generally occurred as a 2-3 day pulse when the upstream ice jam broke free (Figure 5c,d). Concentrations of TSS in both rivers during the summer were typically in the range mg/L
4. Offshore Transport Under Ice (continued) N
Coastal Beaufort Sea with ~2 m thick ice
Sagavanirktok River SPRING flow
(c)
Temperature
Turbidity
Salinity
5.44 mg/L
(a)
(b)
Figure 9. Vertical profiles for (a) salinity and (b) temperature for station S2 in 2006 with depths below ice and (c) mooring data for salinity, temperature and turbidity for station S1 in Station locations are shown on Figure 3.
Figure 1. Aerial photo showing the mouth of the Sagavanirktok River and ice-covered Beaufort Sea during spring flood, May 2001.
(a)
(b)
(c)
(d)
% Sagavanirktok River Water at 1 m
Sagavanirktok R.
Kuparuk R.
% Kuparuk River Water at 1 m
May 22 and then a >60% decrease in salinity was observed on May 24 with continued decreases in salinity over time (Figure 9a,b). A continuous record for salinity, temperature and turbidity for station S1 shows that the top layer of the water column contained about 50% river water on May 23, 2006, when the mooring was put in place (Figure 9c). A snapshot of the fraction of Sagavanirktok and Kuparuk river water in the coastal Beaufort Sea in 2004 (Figure 10) shows the >15 Km seaward extent of freshwater movement with convergence of the two river water flows along the interface defined by the red arrow on Figure 10b
(a)
Mountain source
Tundra source
Figure 5. Hydrographs for water flow and concentrations of total suspended solids (TSS) for the (a) Sagavanirktok River in 2001 and (b) 2006 and (c) Kuparuk River for 2002 and (d) 2006.
(b)
Despite the large range in values for TSS, concentrations of particulate metals and organic carbon in the river-borne suspended sediments (per gram dry wt.) were quite uniform during the spring (Figure 6). In contrast, values for dissolved Fe, Pb, Zn, Mn and some other trace metals, along with DOC, often increased by 3- to 25-fold in river water within 7 days of the onset of runoff due to thawing of ponds and upper soil layers (Figure 7). These peak values during peak flow decreased to near baseline values in a few days. Concentrations of dissolved Zn, Pb and DOC were two times higher in the Kuparuk River than the Sagavanirktok River and values for dissolved Fe were 5 times more in the Kuparuk River (Figure 7d)
2. Study Area and Sampling Locations
The Sagavanirktok, Kuparuk and Colville rivers flow into the Beaufort Sea from the North Slope of Alaska (Figure 2). River water and suspended sediments were collected from the rivers during the spring floods of 2001, 2002, 2004 and 2006 with an emphasis on the Sagavanirktok and Kuparuk rivers based on ease of accessibility. Water and suspended sediments also were collected beneath landfast ice in the coastal Alaskan Beaufort Sea with the most extensive sampling expeditions in 2004 and 2006 (Figure 3)
Figure 10. Contours showing the calculated percent of (a) Sagavanirktok River water and (b) Kuparuk River water under ice in the Beaufort Sea for
Figure 2. Map showing study area on the North Slope of Alaska. Blue rectangles show sampling locations and green rectangles show USGS gauging stations.
(d)
(a)
(b)
(c)
800 µM
400 µM
15 nM
10 nM
5 nM
Sagavanirktok River plume May-June 2004
(a)
(b)
Spring versus Summer
(a)
(b)
8 nM
Figure 11. Concentrations of salinity versus (a) TSS, (b) dissolved As and (c) DOC at under-ice stations during spring 2004 and (d) comparison of profiles for salinity during spring (under ice) and summer at the same locations.
Figure 3. Maps showing sampling stations for under-ice sampling periods seaward of the Sagavanirktok and Kuparuk rivers during (a) 2004 and (b)
4 nM
Figure 6. Concentrations of Fe, Pb and organic carbon for suspended particles (per g dry wt.) from the Sagavanirktok River during spring floods
The data sets for suspended sediments and water across the salinity gradient under ice show that TSS does not follow a simple mixing trend as particles are settling out of the surface plume of river water (Figure 11a). Data for dissolved As for all offshore samples show a more conservative trend (Figure 11b). In contrast, the plot for DOC (as well as many other substances) versus salinity is complicated (Figure 11c) by the sharply shifting concentrations of DOC in the rivers (Figure 7a) during the brief study period. During late June and early July, the under-ice plumes mix with ice melt water, subsurface water, river flow from summer rain events, and upwelled offshore water as the open-water season approaches (Figure 11d), and a whole new set of biogeochemical interactions begins
Peak Flow
(c)
(d)
Kuparuk River
Sagavanirktok River
May 21
June 4
June 18
Peak dissolved Fe
2 μM
4 μM
3. Riverine Inputs
As mentioned above, a large fraction of the annual water flow occurs during the brief period of spring floods, with additional inputs during scattered summer rains (Figure 4a). Distinct differences were observed in the annual flow patterns during the spring floods including as much as a 16 day offset in peak flow between 2001 and 2004 (Figures 4b,c)
1 μM
2 μM
5. Conclusions
● Spring floods deliver >80% of the suspended sediment and >50% of the dissolved chemicals to the Beaufort Sea in 2-3 weeks and riverine concentrations of selected dissolved metals and organic carbon often peak during peak flow ● River water is transported >15 Km offshore, under ice, during the spring melt showing transport pathways for freshwater, suspended sediment and dissolved chemicals ● Rivers are an important source of several dissolved trace metals (Fe, Zn, Pb) and DOC to coastal waters; and offshore waters are an important key source of As and other elements such as Cd, N and P
Figure 7. Concentrations of (a) dissolved organic carbon and Pb for the Sagavanirktok River, (b) dissolved Zn for the Sagavanirktok River, (c) dissolved Fe for the Sagavanirktok River and (d) dissolved Fe for the Kuparuk River with blue shaded box showing distribution of peak values for the Sagavanirktok River
Figure 8. Sampling river plumes through the ice in the Beaufort Sea.
>60%
4. Offshore Transport Under Ice
The flow of river water offshore under ice into the Beaufort Sea was traced by collecting vertical profiles for salinity, temperature and turbidity as well as by collecting water and suspended particles for analysis at 28 stations in 2004 and at 15 stations plus 4 moorings in 2006 with replicate sampling at several stations during each year (Figure 8, locations in Figure 3). Vertical profiles for salinity and temperature for station S2 (2006) show a sharp pycnocline along with a time series for the river water plume (Figure 9a,b). No river water was detected at station S1 on
Acknowledgments. We thank Michelle McElvaine, Debra Woodall, Mary Sohn, Gary Lawley, John Hardin and Mike Walsh for assistance with sampling and Mark Mertz for vessel operations. We thank BP for providing access to and laboratory space on the North Slope. We especially thank Dick Prentki of MMS for assistance in the field and continuing discussions and input to the research plan. Financial support for this research was derived from the U.S. Department of the Interior, Minerals Management Service, contract CT
Figure 4. Hydrographs for (a) Kuparuk River for period of flow during 2001 and (b) Kuparuk River and (c) Sagavanirktok River during spring floods of 2001, 2002, 2004 and (Water flow data from