Intermittent Rivers Will Likely Grow In Number As Earth Warms; They're Far Less Hospitable To Life
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What physical and biological changes can we expect when perennial rivers begin drying? It’s unlikely that a perennial river will simultaneously dry over its entire length. Instead, the point in the river where flow is currently lowest will dry first, and the dry reach will gradually expand upstream and downstream as runoff declines and the groundwater table drops. These expanding dry reaches appear annually in naturally intermittent rivers like the Selwyn, Orari, Pareora and Waipara. Dry reaches are impassable barriers to migratory fish such as eels and bullies; mature female eels can’t reach the coast for their ocean spawning migrations and the returning juvenile eels can’t move from the coast to inland tributaries. Larval bullies can’t drift downstream to estuaries or return as adults.
Life in an intermittent river is not benign. At the start of the drying cycle, fish and invertebrates are trapped in isolated pools, which attracts predatory birds. The pools rapidly heat up and then dry, along with their inhabitants. Invertebrates that are capable of burrowing or breathing atmospheric oxygen can survive in dry river gravels temporarily, but are eventually killed by desiccation and heat stress. Aquatic species can only persist in intermittent rivers if they recolonise when flow resumes, and slow colonisation means that intermittent rivers inevitably have fewer species than perennial rivers.
Despite the negative effects of drying, there are also some possible benefits. One is that native fish and invertebrates may find refuge from predation by non-native trout when they are separated from the trout by dry reaches. Species that are highly resistant to drying, such as mudfish, may find refuge from predators in gravels beneath dry reaches. Increased intermittence due to climate change may be a boon for these species.
In some rivers affected by climate drying, complete loss of flow may take decades to occur. In the meantime, what will happen to the life in these rivers? Seasonal low-flow levels will decrease, with a corresponding loss of habitat. Maximum water temperatures will increase, which has the dual effect of increasing metabolic stress and reducing dissolved oxygen levels. Even now, we see fish killed by hypoxia in isolated pools in the Selwyn River. As with flow intermittence, reductions in aquatic habitat and increased water temperatures are likely to benefit some species. Predators take advantage of habitat shrinkage that concentrates their prey. And non-native species that tolerate high water temperatures and low oxygen, including koi carp and mosquitofish, will persist where sensitive native species are lost.
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https://eos.org/articles/dry-rivers-offer-a-preview-of-climate-change