In an event partly blamed on elevated sea levels associated with El Niño, the mouth of the Tijuana River became blocked with sand for two weeks, preventing the normal tide circulation of the ocean and freshwater and resulting in a stagnant pool of sewage-contaminated water.
The river has since been reopened, but the discovered results were disastrous.
The blockage, the first of its kind since 1983, was responsible for killing dozens of leopard sharks, along with striped mullet and pipefish that were left without oxygen, according to the San Diego Union-Tribune.
The natural dam consisted of 500 cubic yards of sand, which also caused the flooding of some nearby streets in Imperial Beach.
Brian Collins, manager of the San Diego Bay and Tijuana Slough national refuges, blamed the devastation on higher sea levels as well as sediment that flowed from the canyons of Mexico into the river and a 2012 sand-replenishing project that deposited 450,000 cubic yards of sand on the shores of Imperial Beach, north of the estuary.
The blockage began two weeks ago when large swells began pushing sand from Imperial Beach southward, CBS 8 reported.
“Waves and tides and currents move material around and, in this case, the swell was just right for this to happen,” Collins told the Union-Tribune.
Not helping matters was the sewage discharge from malfunctioning equipment at the International Wastewater Treatment Plant.
The nutrient-rich water caused a bacteria bloom and a depletion of oxygen, no doubt contributing to the deaths of the leopard sharks and other fish.
Imperial Beach Mayor Serge Dedina counted as many as 50 leopard sharks.
CBS 8 reported that the Fish and Wildlife Service will assess the ecological impact the closure had on wildlife and habitat, though the dead sharks are "clear evidence" that it had a significant impact.
The Tijuana River enters the Pacific Ocean via an estuary, which is said to be a critical habitat for leopard sharks.
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