Spatiotemporal Distributions of Icebergs in a Temperate Fjord: Columbia Fjord, Alaska

Fjord-terminating glaciers account for the majority of recent sea level rise. Inside fjords, icebergs may affect glacier-ocean interactions by cooling incoming ocean waters, enhancing vertical mixing, or by providing back stress on the terminus. However, relatively few studies have been performed on iceberg dynamics inside fjords, particularly outside of Greenland. We examine icebergs calved from Columbia Glacier, Alaska, over eight months spanning late winter to mid-fall using 0.5-meter resolution satellite imagery, identifying icebergs based on pixel brightness. Iceberg sizes fit a power-law distribution with an overall power-law exponent, m, of −1.26 ± 0.05. We find that iceberg calving rate, rather than water temperature, appears to be the major control on the exponent value. We also examine iceberg spatial distribution inside the fjord, and find that large icebergs (10,000 m2 − 100,000 m2 cross-sectional area) have low spatial correlation with icebergs of smaller sizes (correlation coefficients between 0.345 ± 0.132 and 0.490 ± 0.114, compared to 0.809 ± 0.052 − 0.989 ± 0.006 for the highly spatially-correlated smaller icebergs), due to their tendency to ground on the shallows. We estimate the surface area of icebergs in contact with incoming seawater to be 2.8 ± 0.58 × 104 m2. When compared with our estimated terminus surface area, 9.7 ± 3.7 × 105 m2, we expect iceberg impact on the heat content of the incoming seawater to be negligible in this fjord. Additionally, we find mechanical buttressing of the glacier to be negligible due to low iceberg density near the calving front and lack of winter sea ice in the fjord.