Pools, Riffles and Runs
What are they and why are they important?
A diverse mix of flows and depths is important in a river system to help create the variety of habitats for fish and invertebrate life. In meandering gravel bedded rivers this diversity is largely provided by the occurrence of Pool – Run – Riffle sequences.
Figure 1: Plan view of a pool-run-riffle sequence
· A pool is an area of slow flowing, deep water, often on the outside bend of a stream or river
· A riffle is an area of fast flowing, shallow water where the surface of the water is broken from flowing over stones
· A run is a smooth, unbroken flow of water that connects pools and riffles, acting as a transition between the two.
Figure 2: Section through a pool-riffle sequence
Good for nature
Each of these habitats and their specific characteristics (depth, velocity, substrate etc.) cater to particular fish and invertebrate species and help to promote and maintain diverse instream river ecology. The information below shows the preferences some fish have1;
· Rapid/riffle – torrentfish, bluegill bullies, kōaro, alpine galaxias, and upland longjaw galaxias
· Run – juvenile eels, trout, and some galaxiid and bully species
· Pool – adult eels, lamprey, various juvenile galaxiid species, and adult kōkopu.
1 – Based on survey worked carried out by NIWA 2014.
Good for recreation
Pools can have a significant recreational value to people in terms of providing ‘swimming holes’, particularly over the summer months. There are a number of locations in the Wairarapa where a particular pool is virtually a permanent feature and has been popular for swimming for generations. These locations tend to be where the river meets a hard rocky bluff, on an outside bend, and the river bed is eroded down creating a deep pool.
Another popular pastime is fishing, in particular for trout, for which pools – riffles – runs play an important part in providing good feeding grounds and protection.
The location and frequency of Pools – Runs – Riffles can change over time due to natural events and processes; such as floods passing through the river system, which rework gravels by the process of erosion and deposition.
Human intervention also has the potential to alter and affect the nature and frequency of these habitats. Here, in the Wairarapa, river management schemes were set up many years ago with a mandate, from the riverside community, to manage and mitigate flood and erosion hazard. Some of the activities carried out in these rivers, using mechanical equipment to move gravel around the river bed, can create situations where established pools, in particular, are destroyed, or severely diminished to mitigate the effects of bank erosion. Sometimes the effects will be short-lived and other times it may take a longer period of time to re-establish the pool-riffle-run sequence. This is general down to the flows in the river and how long it is before ‘freshes’ or floods come along to re-establish a natural order.
Traditionally this work was carried out on the basis that financial/economic values outweighed other instream, ecological values, however, as local/national values and economic drivers have changed this has led to a shifting emphasis on how these values should be weighed up against one and other. This changing balance of values has been reflected in scheme activities in more recent years but further work is being carried out to promote improvement.
This does not necessarily mean that mechanical intervention in the river should not take place, but it should be planned and managed in such a way that all the values are weighed up and appropriately considered to help deliver good outcomes for the wider river community (in-water and out-of-water).
Continued research into the effects of these types of activities, education and good practices around when to, and how best to, intervene are important in driving improvements.
Tools to achieve good river management outcomes
The Floodplain Management Plan is looking to develop a range of tools to help deliver sustainable, long-term management of flood risk, based on its established vision and aims.
One tool to potentially help offset instream channel work could be to ensure that the sequence if pool-run-riffle is recreated, to some extent, after works have been completed.
Another tool could be to identify a number of pools, runs, riffles that would be indicative of a healthy diverse river system and to try and maintain this number (or more likely number envelope).
Further work needs to be carried out to identify what the most appropriate set of tools should be and how they could be applied.
We’d like to hear you thoughts though so get in touch and let us know what you think.