A river flowing down a steep slope follows a pretty straight path, with gravity exerting a tremendous pull on the water. But a river flowing over a flat landscape can meander left and right, occasionally abandoning river channels to become oxbow lakes or to take a shorter route to the sea.
In the past few centuries, the Horton River in northwestern Canada stopped wandering and assumed a more direct route to the sea. Situated about 420 kilometers (260 miles) east of the Alaska border, the modern Horton River empties into Franklin Bay; but as this image shows, it once followed a different path. The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite acquired this image on July 31, 2012.
The Horton River Delta forms a fan-shaped interruption to the otherwise straight coastline. North of the delta, lakes fill abandoned meanders. Centuries ago, the Horton River continued another 100 kilometers (60 miles) or so northward, draining into Harrowby Bay. In a 1989 study, researchers from the University of British Columbia used multiple lines of evidence to estimate a time frame for when the Horton River abandoned its old channels and adopted a shortcut to the sea.
One line of evidence comes from historical records. Dr. John Richardson of the second Franklin expedition mapped the mouth of the Horton River sometime between 1825 and 1827, and his colleague E.N. Kendall drew a sketch of it. Obviously the river had taken its shortcut to the sea by then.
Next, the researchers turned to tree-ring dating of driftwood to identify the earliest possible date for the shortcut. Near the sea, the Horton River flows through treeless tundra; but about 200 kilometers (125 miles) upstream, spruce trees grow. When pieces of wood wash into the river, they can be carried far downstream, either washing into the ocean, or getting deposited somewhere along the riverbed. That means driftwood lying in the Horton River’s abandoned channels must have been carried there before the river changed course. Analysis showed that the most recent driftwood in the old meanders was deposited before 1640.
The third line of evidence for the delta’s age comes from studies of the rock and sediment layers, including observed rates of delta growth since the area was first mapped. The researchers concluded: “If rates of delta and fan development are assumed constant, then the breakthrough on geomorphic evidence would have occurred about 1750.”
Although the land around the Horton River is flat, it is not at sea level but instead sits about about 75 meters (250 feet) above sea level, and cliffs line the coastline. So once it stopped meandering and broke through a barrier, the Horton River took a steep trip downward to the Canadian coast. The river’s shortcut formed a delta that continued growing for decades.
A comparison between Richardson’s mapping efforts and aerial photographs acquired later indicates that the Horton River Delta expanded between the early 19th century and the mid-20th century. But subsequent aerial photos showed the delta shrinking later in the 20th century. A study published in 1998 concluded that the ocean eventually began to eat away at the delta faster than river sediments could build it.
NASA Earth Observatory image by Jesse Allen and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team. Caption by Michon Scott.