One of the world’s largest deltas is a shining example of how water and ice can shape the earth.
The Yukon-Kuskokswim Delta is one of the largest deltas in the world, and it is an outstanding example of how water and ice can shape the land. These images show the northern lobe of the delta, where the Yukon River empties into the Bering Sea along the west coast of Alaska.
“The Yukon Delta is an exceptionally vibrant landscape, whether viewed from the ground, from the air, or in low Earth orbit,” said Gerald Frost, scientist at ABR, Inc. — Environmental Research and Services in Fairbanks, Alaska. The vivid landscape is captured in these images acquired with the Operational Land Imager (OLI) on Landsat 8 on May 29, 2021. The images are composites, mixing natural color images of water with a false color image of the Earth.
While the image can be viewed as a work of art, there are some useful aspects to looking at the earth in this way. For example, you can easily distinguish areas of living vegetation (green) from bare land or with dead vegetation (light brown) from the network of sediment-rich rivers and flood waters (dark brown). A pinch of thermokarst lakes is also in the game.
In general, the green areas across the delta are tall shrub willows. They are particularly apparent on either side of the river channels in the image detailed above. The light brown areas are mainly wet decay; they appear brown because much of this is leftover from last year’s growth. Far from the delta (right side of the image), the vegetation is a tussock shrub tundra.
“For me, one of the interesting things about the Delta is that it is a highly transitional area, with some elements of arctic tundra and some of boreal forest,” said Frost.
The delta also changes with the seasons. At the time of this image, the signature of the spring floods is inscribed across the delta. Melting snow and ice causes rivers to overflow onto their banks, and by the end of May, many marshes are filled with flood water, which comes in the form of dark brown ponds.
According to Lawrence Vulis, a graduate student at the University of California at Irvine, the delta would have appeared much more flooded immediately after the snow and ice melted a few weeks before this image. Stream gauges and satellite images suggest that most of the flooding has already subsided. Yet the flooding was recent enough that the abundance of puddles persisted on May 29. As the summer progresses, the floodwaters will continue to decrease and the wetlands will continue to green with vegetation.
Also notice the colorful water where the delta meets the Bering Sea. It is a product of glacial runoff far upstream, which carries a large amount of sediment to the coast. These sediments also contribute to the formation of tall “levees” on the sides of the canals, which settle there when floodwaters flow onto their banks. These “levees” support stands of tall willows, important habitat for moose.
“It’s interesting that tall shrubs have grown a lot in the Delta over the past few decades, and moose have followed suit,” Frost said. “Today, the delta has one of the highest moose densities in the state of Alaska.”
The Delta hasn’t always looked that way. Studies have shown that the modern Yukon Delta is only a few thousand years old. It’s a young age “is amazing to think about it,” Vulis said. “We’re used to thinking of relatively old landscapes, but modern river deltas have only formed in the last 10,000 to 8,000 years since sea level stabilized.
The delta could very possibly be different in the future. “The Yukon and other Arctic deltas are believed to be particularly vulnerable to climate change,” Vulis noted, “due to the role of permafrost and ice in the formation of these deltas.”
Images from NASA’s Earth Observatory by Joshua Stevens, using Landsat data from the US Geological Survey.