By Aaron Delonay, Robert Jacobson, and Casey Hickcox
As the temperatures in the Missouri River Basin begin to rise each spring, snowmelt and spring storms typically introduce large quantities of water into the system. This water becomes runoff that enters numerous tributaries that collectively join the main channel of the Missouri River to create a pulse of discharge often referred to as the annual “spring rise”. In most years the mainstem reservoir system stores much of the spring rise, but annual spring rises of 100,000 cubic feet per second (cfs) or more can exist on the Lower Missouri River during extreme runoff years and especially downstream of the Platte and Kansas rivers.
Pallid sturgeon, like many fish species, spawn over a range of temperatures in the spring when seasonal rises in rivers occur. Based upon observations of our telemetry tagged sturgeon, most spawning in the Lower Missouri River occurs between 16-22 °C (61-72 °F). For pallid sturgeon, water temperatures appear to be the primary influence on when and for how long spawning activity occurs during the spring. Seasonal flow changes and river discharge may also influence pre-spawning behaviors, particularly spring migration, and may play a role in determining the precise timing of adult aggregations and spawning events.
During much of this year’s reproductive window (the time when sturgeon are expected to spawn), the Lower Missouri River near Boonville, Missouri, maintained a relatively low flow with only one small pulse that added about 30,000 cfs (figure 1). In areas upriver near Sioux City, Iowa, flow was low and even less variable through the reproductive window (figure 1). Only after the expected peak of pallid sturgeon spawning activity had passed did the Missouri River near Booneville see substantial increases in flow as the river rose 13 feet and discharge increased by over 100,000 cfs in less than 4 days. At Sioux City, meanwhile, cooler water temperatures extended the expected spawning period for pallid sturgeon to early June. A spring rise peaking at over 80,000 cfs occurred in late June due to heavy rainfall in the Big Sioux River, however this pulse was probably too late to influence sturgeon migration and spawning.
Pallid sturgeon have evolved in a river with naturally variable flow and temperature, but management of the Missouri River also creates conditions that are outside of the range of natural variability. CSRP research addresses the question of how well sturgeon can adjust to unnatural variability and still maintain a sustainable population. Long-term studies that include the kind of variability experienced with the extreme flows of 2011, the drought of 2012, and the lack of an early spring rise in 2014, help to answer this question.