The Dry Reach, An Impassable Barrier
A Science Spotlight
By Steven Esau, Water Quality Technician (BC)
September 23, 2023
Every summer, a curious thing happens to the Tatalu / Little Campbell River. In its middle reaches, the river stops flowing, becoming a string of isolated pools which gradually dry out entirely. This dry reach (see map below) persists for around four months out of the year, even while the river is flowing both up and down-stream of it. It has profound implications for the surrounding wildlife. Every year, thousands of coho salmon fry are stranded in the pools, eventually dying due to desiccation or lack of oxygen. In the fall, the dry reach is an impassable barrier to spawning salmon, cutting them off from the upper half of the river. They must wait until the river reconnects, which can happen as late as November.
The Way of Water
Water flow in the Tatalu is intimately tied to groundwater. In some places, groundwater emerges to feed the river; in others, river water seeps in to recharge the groundwater. How much water and in which direction depends primarily on the height of the riverbed relative to the water table and the permeability of the channel1.
During the summer, the water table drops and there is little precipitation. In certain areas of the river, the amount of water seeping into the ground exceeds the amount of flow coming from upstream, so there is no water left to continue downstream—therefore the river disconnects. Differing conditions in other areas allow them to stay wetted year round.
Increasing Ecosystem Stress
Intermittent rivers and streams (i.e. those which stop flowing in some areas, for some parts of the year) are extremely common and certainly part of the natural landscape2. There are aquatic species that are adapted to resist desiccation, while other terrestrial organisms benefit from new habitat in the dry riverbed2. But globally, dry reaches are becoming more prevalent, often growing in size and duration as a result of human impacts1. These changes expose more organisms to drying and push others beyond their adaptive limits.
Along the dry reach of the Tatalu, there are many anthropogenic factors at play. The water table is lowered by water extraction from the wells nearby (it is worth mentioning that we at the A Rocha BC Centre also get our water from a well near the Tatalu). More of the surrounding area is being paved, which prevents precipitation from soaking into the ground. The topography of the area was altered extensively in the past by gravel extraction.
Lastly, climate change increases the likelihood of drought. These factors can introduce drying to new areas and accelerate and extend it in others.
Walking the River
In 2018, our conservation science team started walking the river bed to record the timing and extent of the dry reach. We’ve repeated this every year since and used the results to raise awareness among community members and local governments. We’ve collaborated with partner organizations to rescue thousands of stranded coho salmon fry (e.g. news article 1, 2). This year, we’re collecting more detailed data to identify potential locations for future salmon rescue and other management possibilities.
These challenges mirror those seen across much of BC. Our A Rocha colleagues working near Houston, BC recently relocated many adult Steelhead trout due to low flows in Buck Creek—a little different than moving coho fry, but done in much the same spirit. There are many individuals and organizations who care deeply about their local rivers and are stepping up to help in times of drought. These rivers are complex systems, from the surfacewater-groundwater interchange to the balance between the needs of different water users. But we hope and pray that what we’ve learned by studying the dry reach will help us and others care for the Tatalu.
Featured photo: Laura Tsai