Port Isabel was a seaport established on Port Isabel Slough in 1865 during the American Civil War in Sonora, Mexico in the mouth of the Colorado River on the Gulf of California. It was founded to support the increased river traffic caused by the gold rush that began in 1862 on the Colorado River and the Yuma Quartermaster Depot newly established in 1864 to support the Army posts in the Arizona Military District. The slough was discovered in 1865 by the Captain W. H. Pierson of the schooner Isabel, that first used the slough to transfer its cargo to steamboats safe from the tidal bore of the Colorado River. Shortly afterward Port Isabel was established 3 miles up the slough and replaced Robinson's Landing as the place where cargo was unloaded in the river from seagoing craft on to flat bottomed steamboats of the Colorado River and carried up to Fort Yuma and points further north on the river.