15.6.2 Case study: Arctic megadeltas
Numerous river deltas are located along the Arctic coast and the rivers that flow to it. Of particular importance are the megadeltas of the Lena (44,000 km2) and Mackenzie (9,000 km2) rivers, which are fed by the largest Arctic rivers of Eurasia and North America, respectively. In contrast to non-polar megadeltas, the physical development and ecosystem health of these systems are strongly controlled by cryospheric processes and are thus highly susceptible to the effects of climate change.
Currently, advance/retreat of Arctic marine deltas is highly dependent on the protection afforded by near-shore and land-fast sea ice (Solomon, 2005; Walsh et al., 2005). The loss of such protection with warming will lead to increased erosion by waves and storm surges. The problems will be exacerbated by rising sea levels, greater wind fetch produced by shrinking sea-ice coverage, and potentially by increasing storm frequency. Similarly, thawing of the permafrost and ground-ice that currently consolidates deltaic material will induce hydrodynamic erosion on the delta front and along riverbanks. Thawing of permafrost on the delta plain itself will lead to similar changes; for example, the initial development of more ponded water, as thermokarst activity increases, will eventually be followed by drainage as surface and groundwater systems become linked. Climate warming may have already caused the loss of wetland area as lakes expanded on the Yukon River delta in the late 20th century (Coleman and Huh, 2004). Thaw subsidence may also affect the magnitude and frequency of delta flooding from spring flows and storm surges (Kokelj and Burn, 2005).
The current water budget and sediment-nutrient supply for the multitude of lakes and ponds that populate much of the tundra plains of Arctic deltas depends strongly on the supply of floodwaters produced by river-ice jams during the spring freshet. Studies of future climate conditions on a major river delta of the Mackenzie River watershed (Peace-Athabasca Delta) indicate that a combination of thinner river ice and reduced spring runoff will lead to decreased ice-jam flooding (Beltaos et al., 2006). This change combined with greater summer evaporation, due to warmer temperatures, will cause a decline in delta-pond water levels (Marsh and Lesack, 1996). For many Arctic regions, summer evaporation already exceeds precipitation and therefore the loss of ice-jam flooding could lead to a drying of delta ponds and a loss of sediment and nutrients known to be critical to their ecosystem health (Lesack et al., 1998; Marsh et al., 1999). A successful adaptation strategy that has already been used to counteract the effects of drying of delta ponds involves managing water release from reservoirs to increase the probability of ice-jam formation and related flooding (Prowse et al., 2002).