Fish Spawning and Migration
As noted previously, dams have wide-ranging effects on downstream habitat through changes they cause to water turbidity and sedimentation and erosion patterns. These changes threaten certain species that have evolved to thrive in the natural system – like the humpback chub – through a combination of decreased or degraded breeding habitat and increased predation by non-native species. Additionally, because water released from dams through intakes flows from the deep part of the reservoir, it is commonly colder than the natural river flow – and its temperature is less variable than in the river’s natural state. For example, prior to construction of the Glen Canyon Dam, water temperature in the Colorado River varied from ~0 to 27° C over the course of the year; water discharged from the dam now averages ~8°C and varies little seasonally. The changes in water temperature and its variability impact some fish species, which rely on temperature cues to trigger key lifecycle events. The lower oxygen levels in waters released from storage behind dams also impact fish downstream.
Finally, dams present physical barriers to catadromous and anadromous fish species (those that spawn in saltwater and live in freshwater, and vice-versa, respectively). These fish migrate either upriver from the ocean (anadromous), or downriver to the ocean (catadromous) to spawn. After hatching, the young fish migrate in the opposite direction. Of these, perhaps the best known is the salmon, which migrates up-river to spawn, commonly over hundreds or thousands of km – for example, although greatly reduced due to major dams along the Columbia and Snake Rivers, Chinook salmon runs commonly extend from the Pacific Ocean all the way to Idaho! Structures designed to allow fish to navigate dams, such as fish ladders, are one solution, but they still present a barrier that reduces the likelihood of safe passage, and thus fish numbers.