I immigrated to New Zealand in 2001 to work at Niwa. That September, I made my first trip down the Selwyn River, from the headwaters near the Rakaia gorge across the Canterbury Plains to Te Waihora/Lake Ellesmere. The river was in flood, carrying mud, willow branches, irrigation hose and uprooted fenceposts to the lake. In a fever of scientific zeal, I launched an investigation of the effects of floods on river ecosystems.
I returned to the Selwyn in November to ask local farmers for access to sampling sites across their paddocks. The first site I visited had no water – just a gravel-lined channel on the flat plains. The second and third and all remaining sites were also dry. That was an unnerving day.
It took several more field trips to grasp that the Selwyn is a naturally intermittent river. When river flow from the headwaters reaches the plains, it seeps down through the coarse gravel bed to the water table 10-20 metres below the surface. Rapid seepage leads to the loss of all surface flow within 5km. A talk with any of the farmers I visited in November would have informed me that most of the Selwyn is dry for most of the year. But I was new to New Zealand and didn’t know to ask.
Since the dry riverbed characterises the Selwyn River more often than floodwater, we shifted the focus of our research from the effects of flooding to the effects of drying. The study lasted six years, kept a team of scientists and university students gainfully employed, and greatly expanded our understanding of intermittent rivers.
There were two big revelations for me. First, drying has pervasive effects on every ecological variable we thought to measure, from biodiversity to water quality. Second, naturally intermittent rivers are giving us a preview of the effects of climate change.
In areas of New Zealand that are undergoing drying trends, many rivers that are currently perennial will become intermittent. Summer low flows are already trending downward in some perennial rivers and will eventually reach zero. Alteration of flows in these rivers by storing and returning irrigation water may modify these trends, but they are inevitably downward in drying areas.
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.
—Scott Larned, chief scientist freshwater for Niwa
This story originally appeared in stuff. It is republished here as part of Eos’s partnership with Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.
Larned, S. (2019), Dry rivers offer a preview of climate change, Eos, 100, . Published on 19 September 2019.