Case Study - The Selwyn River

The Selwyn river has long been a distinctive feature of the Canterbury region’s acclaimed natural landscape.

Fed by rain in the foothills and small springs in the lower plains, it snakes across a patchwork of Canterbury farmland on mostly wide, porous shingle beds before entering the serene waters of Lake Ellesmere.

Once known as an angler's delight, the river has undergone dramatic changes in flows and ecology in the past century, a subject of considerable concern and debate in the region.

Let's take a look at the history of the Selwyn, how it has been affected by human activities, climate change, and natural processes, and what can and is being done to restore it to its former self.  

A Brief History of the Selwyn

Before the arrival of European settlers in New Zealand, the Selwyn was known only as Waikirikiri, a Maori term which translates, appropriately,  to 'river of gravel and stones'. 

That changed in 1849, when British explorer Captain Joseph Thomas was appointed chief surveyor by the Canterbury Association, and set about naming and documenting a number of natural features in the region, including the Waikirikiri, which he called the 'Selwyn River' after Bishop George Selwyn of Lichfield (a name he chose for the surrounding district as well). 

Thomas mapped the Selwyn's full 80-kilometer length from its source in the Southern Alps, out of the foothills, across the Canterbury plains, and right through to its termination at the picturesque Lake Ellesmere/Te Waihora, south of Banks Peninsula. 

A spokesperson for Ecan gives this succinct description of the physical geography of the river:

“Essentially the Selwyn can be split into three different sections.

“Firstly, there are the headwaters up above white cliffs, which flow all the time. Then, the Selwyn comes out of the foothills and runs across the Canterbury plains, losing water the whole way to groundwater and drying up naturally across the plains (it will flow at times, but across the summer months it will almost always dry up in some place across the plains). Then, from about state highway one down, you get water re-emerging and the Selwyn becomes like a spring-fed stream.

So, you have the upper, a foothill stream which runs all the time, you’ve got across the Canterbury plains which flows some of the time and you’ve got the bottom bit which flows all of the time as a groundwater fed system. But, what varies is the amount of time and the geographic length for which it is dry. It varies in terms of space and time across those three different bits.”

Brown trout were introduced to the river in the 1860s, and the bountiful Lake Ellesmere fishery quickly followed.

Aside from trout, the river was also home to several native fish species, including Inanga, Koura (crayfish) and the Longfin Eel. 

 Table: fish species of the Lake Ellesmere/Te Waihora system and the Selwyn River

Table: fish species of the Lake Ellesmere/Te Waihora system and the Selwyn River

The latter, along with its shortfin cousin, would first be fished commercially in the early 1970s, with the establishment of the Lake Ellesmere Eel Fishery, which by 1976 had become the largest single fishery in New Zealand. 

However, from the 1980s onward, the Ellesmere fisheries experienced a sharp, unrecoverable decline, evidenced by the fact that in 1985 just 40 trout were trapped at Coes Ford moving up the Selwyn river to spawn, an incredible 99.97% reduction from the 14,000 trapped 20 years earlier in ‘66.

The decline of the fisheries would coincide with a marked reduction in water quality and flows, widely acknowledged by the beginning of the 21st century.

This first became a matter of real public concern in 2010, when Canterbury health authorities issued a warning after potentially toxic blue-green algae was found in the Selwyn River/Waikirikiri at Glentunnel camp site.

 Above: The Selwyn circa 1960  Below: The Selwyn in the summer of 2017, low and choked with algae

Above: The Selwyn circa 1960

Below: The Selwyn in the summer of 2017, low and choked with algae

Since then, a stream of articles and news pieces bemoaning the state of the Selwyn have surfaced. In 2017 alone, more than thirty articles were published bearing headlines like: “Selwyn’s water woes becoming worse and worse”, “Sixty percent dip in Selwyn River flow affecting trout population” and “No longer swimmable: A community mourns its lost river”.

How did the Selwyn reach this crisis point?

Before we get into that, let’s take a more detailed look at the river’s once thriving fish population.

The Selwyn/Te Waihora-Ellesmere fisheries

Previously, the Selwyn River was lauded as a fantastic fishing spot, with a spawning run said to exceed 60,000 fish per season, and anglers coming from all over the country to experience its brilliantly clear waters and large, plentiful trout population.

One can look as far back as W.H Spackman’s 1892 guide, “Trout in New Zealand: Where to Go and how to Catch Them” to find descriptions of the “prettiest sport” of fly fishing at the upper and lower Selwyn, with anglers said likely to bring home trout-filled baskets “weighing as much as 15 to 30lb”.

Brown trout had first been introduced to the Te Waihora/Lake Ellesmere system in 1868, and a thriving fishery, considered by many the best in all of New Zealand, was soon established.

Unlike other Canterbury lowland fisheries, which relied on ocean production, the Te Waihora/Ellesmere system saw hugely successful production within the lake itself. This was put down to a few key factors, including the weed beds which were thought to provide excellent shelter, protection and feeding opportunities for young trout, and the ideal nature of the Te Waihora/Ellesmere tributaries, in particular, the Selwyn.

As summer drew to a close, the brown trout of the Te Waihora/Ellesmere system would move upstream out of the lake and into the lower reaches of tributary streams to spawn.

Alan Strong, in his essay, “The Te-Waihora/Lake Ellesmere Brown Trout Fishery “Collapse and the Short Term Recovery” describes this Autumn migration process:

“The main spawning run of trout would occur in Autumn with several thousand fish migrating up the Selwyn River to access the spawning grounds up river. Once the spawning was over the adult fish would drop back down the Selwyn to the lake where they were safe and could recover condition with an abundance of food available. The hatched fingerlings yearling trout would quickly fill the available space within the river ecosystem, the balance being pushed down the river to the lake. In the lake, they could live and grow until they matured and would then repeat the cycle. These migrations, naturally, required adequate flow for cyclical fish passage.”

It was this access to permanent water in the foothills that separated the Selwyn from the other spawning streams and is why estimates of the size of the Te Waihora/Ellesmere fishery were taken from measurements of spawning trout in the Selwyn’s lower reaches.

These estimates reportedly reached their peak in the 1940s, with population numbers reaching as high as 65,000 trout.

The system, in fact, grew to a point of such productivity that it was, for a time, used as a hatchery for other fisheries, and was said to contain enough fish to “stock all the rivers in Southland”.

Naturally, the thriving trout population of the Selwyn made it tremendously popular amongst anglers. Settlers had dubbed it “the best 3 miles of brown trout fishing in the dominion” back in the late 1800s, and for much of the next century, the Te Waihora/Lake Ellesmere would remain one of the most heavily fished river systems in the country.

 Mr A. W. Sparrow and Mr O. von Sierakowski (1924) each sporting five pound bags of brown trout taken from the Selwyn. Thousands of anglers flocked to the river annually in the first half of the 20th century.  (Image Credit: Christchurch City Library)

Mr A. W. Sparrow and Mr O. von Sierakowski (1924) each sporting five pound bags of brown trout taken from the Selwyn. Thousands of anglers flocked to the river annually in the first half of the 20th century.

(Image Credit: Christchurch City Library)

Its popularity was furthered in the early 1970s with the establishment of a commercial eel fishery, which by 1976 comprised half of New Zealand's total eel catch. Substantial longfin numbers were recorded in the lower reaches of the Selwyn throughout the decade, however, the size of the annual catch from the fishery gradually began to decline. 

In reaction to this, Lake Ellesmere was declared a 'controlled fishery' in 1978 and the minimum size limit was gradually increased from its original 150g to 220g to promote a boost in the population. The total allowable catch (TAC) was also adjusted, in 1986, lowering from the original 256 to almost half that number (136.5).  

A concession area and period were introduced at Taumutu in 1996, which allowed fishers to target migrating shortfin males that were less than legal minimum size, a measure intended to reduce the pressure on the total allowable commercial catch (TACC). This was considered an effective method of conserving females and utilising the extensive common bully population of the lake which formed the major prey of larger eels.

The targeting of migrating males caused a marked decline in their size over time, though this decline has reportedly been arrested in recent years.

The overall eel catch at the fishery, however, gradually lessened, as did that of the trout fishery, which, from the time it reached its peak in fish numbers in the 1940s, had been in a rapid state of decline.

In fact, according to Kenneth Hughey and Kenneth Taylor, editors of "Te Waihora/Lake Ellesmere: State of the Lake and Future Management",  it is now one of the most degraded fisheries in New Zealand, having experienced unprecedented changes in the past half-century.  

Hughey and Taylor break this decline into two distinct phases, the first occurring directly after the Wahine Storm of '68, the second in the past 20-30 years. 

Post-Wahine Storm: the first phase of the Selwyn’s decline

The Wahine Storm of 1968 is considered the second largest to hit New Zealand in the 20th century. Its gale force 150 km/h winds caused havoc across the country, most infamously at Wellington Harbour, where 54 lives were lost in the tragic Wahine Disaster. 

These powerful winds severely impacted the Ellesmere system, ripping large beds of macrophytes (aquatic weed - mainly ruppia and potamogeton pectinatus) from the lake bed overnight.

Weed beds were a key factor in the systems ability to sustain large populations of thriving fish, providing food, shelter from anglers and the direct heat of the sun, and acting as a wave barrier, reducing lake edge erosion and sediment re-suspension.

The effect of this loss was obvious and immediate; Between 1966 and 1976, the number of spawning fish at Coes Ford dropped from 14,000 to 2,000. However, Alan Strong recalls the fishing still being “very good” on the Selwyn for a time, with anglers continuing to arrive in high numbers. He attributes this to the fact that the weed beds of the Selwyn, unlike those of the lake, were relatively unharmed during the Wahine Storm, and this facilitated a small recovery between 1977 and 1981, where the number of spawning fish increased gradually from 2,000 to 3,000.

Unfortunately, this recovery would be short-lived.

Second phase of decline

1982 is considered by many the year the Selwyn took a turn for the worse.

Alan Strong describes the occurrence of a phenomenon in Lake Ellesmere that facilitated this turn:

“In 1982 we were first told of the term eutrophication, and that the lake had flipped from a weed-dominated environment to an algae-dominated one. Effectively, the lake was polluted; high levels of nutrient (Nitrogen and phosphorous among others) were enabling out of control algae growth.

It is likely this situation had been building for some time until the lake system reached a tipping point, or flipping point as it is known. This would have been exacerbated by the loss of the weed beds which absorbed nutrients, including nitrogen and phosphorous, removing them from the water and limiting algae growth.”

The removal of weed beds and subsequent eutrophication are only two of several factors contributing to the deterioration of Lake Ellesmere and its tributaries. Others include:

  • unstable lake level control

  • commercial net setting

  • reduced river flow due to over-abstraction

  • nutrient leaching and runoff from farm land

  • loss of spawning areas

  • predators and pests, and;

  • recreational over-fishing

Each of these areas of concern has been evaluated and targeted for improvement under the Canterbury Water Management Strategy and Plan Change One.

The CWMS: addressing concerns over ecology and water quality in the Selwyn

The Canterbury region relies heavily on water to drive agricultural productivity, tourism and industry.

cant water m strat.jpg

In 1998, in order to maintain control over the take and use of this precious resource, which was increasingly coming under pressure, Environment Canterbury, the Ministry of Agriculture and Forestry and the Ministry for the Environment initiated the Canterbury Water Management Strategy (CWMS).

A key aspect of that initiative was the division of the Canterbury region into 10 water management zones, each with its own management zone committee tasked with consultation, assessment and decision making responsibilities.

The Selwyn River is located within the Selwyn Waihora water management zone, which stretches from the upper Waimakariri basin and the high country around Lake Coleridge down to Te Waihora/Lake Ellesmere.

In 2014, the Selwyn-Waihora Water Management Committee devised a variation to the Canterbury Land and Water Regional Plan ( a region-wide plan developed under the Canterbury Water Management Strategy with the goal of addressing a wide range of land and water-related issues), putting in place policies and rules to help achieve the communicates goals for freshwater. This variation, named ‘Plan Change One’ officially came into effect in 2016, and identifies a range of issues affecting flows and water quality in the Selwyn catchment, a big one being nutrient leaching.

Nutrient Leaching/runoff

As Strong explains, the Ellesmere and its tributaries are nutrient rich, and have been for a long time. This was less of an issue when the system’s weed beds absorbed these nutrients (mainly nitrogen and phosphorous) but has, in the past decade, become a more serious topic of discussion.

As explained by Canterbury water in “Selwyn Te Waihora: Our Water Story”, nitrogen and phosphorous increase pasture and crop growth when applied to farms strategically, however, in excess, they can seriously impact the quality of surrounding water bodies, leading to nuisance growths of slime and algal blooms. The Selwyn has felt these effects as farming has intensified in the region (the number of dairy cattle in Canterbury rose by 19% between 2011 and 2012 and has more than doubled since 2002), facilitating a rise in levels of nitrogen and microbial contamination.

Indeed, data collected between 1993 and 2007 shows the four sites on Te Waihora/Lake Ellesmere to have greater nitrogen concentrations than those of other coastal lakes in Canterbury, placing them in the hypertrophic (term describing lakes/bodies of water with excessively enriched nutrients) category.

To address this point of concern, plan change one has introduced a number of stipulations around farm nutrient limits in the Selwyn district:

  • Firstly, from 2014, farmers with nitrogen losses greater than 15 kg of nitrate per hectare per year were required to achieve good management practice nitrogen loss rates for their existent land use.

  • From July 2017, any farmers within the ‘Cultural and Phosphorous and Sediment Areas (unique to the Selwyn Waihora catchment) were and are required to have a farm consent

  • From 2022, farming activities in the area must reduce their nitrogen loss, in the case of dairy, by 30%.

Additionally, stock access to drains has been prohibited and farm environment plans targeting how farmers will engage in practices that reduce the harmful effects of their activities have been introduced.

Abstraction for irrigation

Abstraction is another key area of concern for the Selwyn/Te Waihora zone.

As Alan Strong says, since the 1970s, the amount of water removed from rivers and directly from aquifers below ground in Canterbury has dramatically increased.

Currently, around 450,000 hectares of land is irrigated in the region, a number that is only expected to increase. The high volumes of groundwater abstracted to meet the demands of irrigation have had a significant effect on river flows, particularly in the Selwyn.

Data from the eight-year period between 1998 and 2006 shows the 7 day mean average flow of the Selwyn River to have dropped from 700l/s to 280l/s. Though a range of factors influenced this change, it has been widely acknowledged by farmers, and Irrigation NZ themselves, that the over-abstraction of water for irrigation has played a part in those reduced flows.

Talking to Radio NZ in January of last year, Irrigation NZ Chief Executive Andrew Curtis conceded irrigators “all acknowledge there are issues with the Selwyn river and Te Waihora” and recognise that groundwater in the river “has been over-allocated in the past”.

However, he also noted a lot of work has been put into rectifying that situation in the past 10 years.

For instance, under Plan Change One, Central Plains Water have so far swapped 10,000 ha of irrigated land off groundwater and on to the stored alpine water out on Lake Coleridge. This is an ongoing initiative expected to yield excellent results over the next decade, “gradually”, as Curtis puts it “readdressing the water balance and putting it back to its natural state”.

A number of strictures apply within this transferal process, an example being: if a farmer wishes to transfer their water to another property within the same groundwater allocation zone, they are required to surrender 50% of their allocated take.

This combination of reduced allocated takes and a swap from groundwater to alpine water is hoped to be a sustainable, long-term strategy for restoring flows at the Selwyn. Significant results are yet to be seen showing just how much of an impact they have had or may have in the future.

Siltation/salinity

A slight overall reduction has been seen in nutrient loadings to the Te Waihora Lake in the past few years. This doesn’t, however, reflect a reduction in nutrient levels in the Lake’s tributaries, rather a reduction in the flows of those tributaries, including the Selwyn.

The lower section of the Selwyn becomes slow flowing and widens as it enters the lake. This lower, slower section of the river can act as a sediment trap for debris during floods. As Alan Strong explains, “the water velocity drops off and the river becomes larger, [meaning] the sediment can no longer be held in suspension and drops out of the flow to the river bottom.”

This siltation (much of which comes from farm runoff) is made worse in the Selwyn by eroding banks and the continued widening of the river (it is now 100% wider than in the 1950s according to Strong).

So, effectively you are left with a wide, shallow silted river bed - a poor environment for fish, and an environment that facilitates increased water temperatures and algal growth.

Strong firmly believes dredging to remove the sediment from these sections of the Selwyn would be of great benefit to the river and it has been recommended several times in other reports on the Selwyn. However, as of yet, nothing has been done about it.

Factors affecting the fish

As previously mentioned, the Selwyn fisheries began to take a dive in the early 1980s, and they have never fully recovered.

Between 1980 and 2006 a 63% reduction occurred in total available spawning area for trout. That situation has continued to worsen and it is believed that area may have been reduced as much as 90%. The trout population is at record lows. A spokesperson from Ecan says numbers that were “once in the tens of thousands are now literally in the tens”.

Low flows due to over-abstraction have certainly played a part in this, making it difficult for trout to reach spawning areas.

Commercial fishing, too, has played a part, though to what degree it is difficult to say, as Alan Strong explains:

“Lake Ellesmere maintains a commercial fishing industry, primarily flounder and eels. Eels are caught using fluke nets… this type of netting does not appear to have a detrimental impact on trout. Flounder fishing is carried out by gill netting which does catch trout. Evidence from the trap at Coes Ford showed up to 6% of fish had net marks from encounters with gill nets.

“A commercial fisherman commented to me that trout by-catch is rare and he actively avoided areas where this may happen, as large trout rip holes in his nets.

“This issue has been raised as being a main contributor to the loss of trout in the lake. While it has undoubtedly had an impact, I do not consider it was a major contributing factor in the latter phase collapse… data must be collected from commercial fisherman, which can be used to assess the population of trout in the lake. If the numbers are such, then a change in net setting practice may be required.”

Eel and bully number have dwindled too, though it is difficult to know by exactly how much. In 2017, following three years of dry winters, Environment Canterbury appealed for volunteers to help native eels and other fish species trapped in the mud.

Fish & Game officer Steve Terry described at the time how drastic a decline the river’s fish population had faced in a very short time-span:

“Three years ago we salvaged well over a thousand trout, and similar number of eels and bullies. Last year we found 100 and this year we're down to the odd fish."

 A longfin eel rescued from the Selwyn River by a DOC ranger in December of 2016

A longfin eel rescued from the Selwyn River by a DOC ranger in December of 2016

Many said the river was worse in 2017 than it had been for forty years. To what extent each individual factor contributed to this it is hard to say with conviction, but there is no doubt that climate change, intensive farming, abstraction for irrigation and commercial fishing are all issues facing the Selwyn, and they must be addressed.

Non-Human factors affecting the Selwyn

Though human activity (fishing/farming) has played a significant role in the deterioration of the Selwyn, it’s important to outline the natural geological characteristics that also influence its low flows, toxicity and reductions in ecology.

The Selwyn is an ephemeral, alluvial plain river, meaning it naturally doesn’t flow for much of the year.

S.T Larned, writing in “River Research and Applications”, explains:

“Hydrological properties that characterize the Selwyn include strong surface water–groundwater interactions, contiguous ephemeral, intermittent, perennial‐losing and perennial‐gaining reaches and an expanding and contracting dry segment that persists for most of the year.” 

It’s important to keep in mind that human activities like farming exacerbate and worsen these natural geological states, rather than being their root cause.

Global warming must also be considered under this category, though perhaps could be termed an indirect human cause of the river’s deterioration. Temperatures have increased worldwide by almost two degrees in the past century, a development that is obviously going to affect water bodies. How much it has troubled the Selwyn is difficult to measure, but the general consensus is that it’s important the long-term effects of climate change and sea level rise on the river be investigated.

The future of the Selwyn

The modern history of the Selwyn has involved dramatic fluctuations in health and its current state is precarious.

Years of commercial fishing, the intensification of farming and irrigation, global warming, and the Wahine storm of ‘68 have all had a hand in reducing the flows and water quality of the river, and its aquatic inhabitants have suffered as a result.

While it’s difficult to provide a complete and accurate statistical representation of the river’s changes, here are a few pieces of data (some of which were mentioned above) showing different levels of deterioration in the Selwyn over the last hundred years:

  • Between 1998 and 2006, the weekly mean average low flow at Coes Ford dropped by 60%

  • Between 1941 and 2007, trout spawning numbers in the same section dropped from over 12000 to less than three figures

  • Between 1977 and 2006, the total eel catch (in tonnes) decreased from over 800 to less than 150.

  • Since the 1980’s trout spawning areas have reduced in size by an estimated 90%

However, the good news is that restorative measures are underway to address these concerns.

The initiation of Plan Change One was an extremely important, very positive step forward for the Selwyn/Te Waihora zone, putting in place specific policies and strictures aimed directly at the aspects of the catchment in greatest need of help.

It is now a matter of allowing these processes time to develop and produce results, hopefully with the outcome of someday restoring the Selwyn to its former state.