Stream enhancement in ChristchurchAlastair Suren

Eric Graynoth

Barry Biggs

Shelley McMurtrie

Rachel Barker

STREAM RESTORATION

Streamrestoration inChristchurch isturning drainsinto livingstreams.

Stream cleaners atwork in Christchurch,circa 1920. (Source:Wilson, J. 1989.Christchurch: Swampto City. Te WaihoraPress, Lincoln)

Alastair Suren, EricGraynoth and BarryBiggs are based atNIWA, Christchurch;Shelley McMurtrie isa consultantecologist with EOSEcology Ltd; RachelBarker is with theChristchurch CityCouncil Parks &Waterways Unit.

12

NIWA Water & Atmosphere 10(2) 2002

Have you ever thought what happens when itrains in a city? Rainwater and oily road grimemix into a greasy cocktail, causing trafficaccidents on slippery roads. Water and debrisquickly flow off impervious roads, carparks androofs. Stormwater drains can become blocked,causing localised flooding. Stormwater is oftenconveyed to small streams, which run dirty asthe water races to the sea. Flood flows in citiesare much higher than in rural streams becausecities shed water rapidly from their hard asphalt,concrete and iron surfaces.

To cater for increased flood flows, urban streamshave been extensively modified into efficientdrains by straightening, and lining with woodor concrete. This is especially true in cities suchas Dunedin, Nelson and Auckland, wherenaturally steep slopes further increase watervelocity during floods.

Other urban streams have been modified fromwhat was originally swampland. The City ofChristchurch, for example, was established in

the mid-1840s in aswampy area, drained byrivers that ran “crystalclear”. These rivers wereimportant mahinga kai forNgai Tahu, who gatheredeels and inanga for food,and who made fibre fromflax bushes that grewalong the banks. By 1875,

however, stream water quality had severelydeclined. An epidemic of typhoid fever thatswept through Christchurch was partly due tolack of a proper sewage scheme, but alsoreflected the soil’s swampy nature.

In response to public pressure, the ChristchurchDrainage Board (CDB) was created in 1875 toremove superfluous subsoil and stormwaterfrom the city, and to dispose of sewage. To fulfilthis task, most of the meandering headwaterstreams of the Avon and Heathcote rivers werechannelled and lined with wooden box-piling.Gangs of “stream cleaners” removed instreamvegetation to maintain high drainage efficiency.

Although the modifications had the intendedresult, the drainage channels provided poorhabitat for the native plants and animals thatoriginally lived there.

In general, urban streams provide little shelter:animals and plants are easily washed awayduring floods and there is no cover for fish frompredators such as birds. There is also often littlevegetation along the banks – either buildingsand carparks encroach right to their edges, orstreamside vegetation is cleared. Lack of shadecan lead to thick growths of algae or waterplants. In summer, low flows can lead to highwater temperatures and declining oxygenlevels. As a result, urban streams often supportlimited stream life – perhaps just a few eels, andtolerant invertebrates such as snails, midgesand worms.

A new view of streamsDuring the 1990s, the Christchurch City Council(CCC) commenced a series of riverenhancement projects throughout the city,reflecting a change in attitude on managingurban streams. The original CDB dogma that“drainage was everything” has been drasticallyrevised and a new focus placed on a stream’secological functions. Projects to improve theaesthetic and ecological values of streams arenow commonplace in Christchurch.

An essential part of the restoration process hasbeen to compare stream conditions before andafter treatment. As part of this monitoring, weselected short reaches (50–100 m) of five streamstargeted for enhancement projects. Riparianplanting was planned for Nottingham Stream,

Jacksons Creek beforestream enhancement...

Urban stream restoration: an essential economy?In New Zealand – as in other regions such as the UK, Denmark andNorth America – there is increasing interest in stream restoration aspeople place greater value on healthy, productive river ecosystems. Todate, most effort has focused on restoring the riparian (streambank)zone in both rural and urban streams.

There have been few attempts to restore habitat structure in NewZealand urban streams. This may reflect low recognition in the past of theimportance of good habitat to support healthy ecosystems, a perceptionthat habitat restoration is very costly, and perhaps a lack of technicalknowledge on how to implement restoration.

But this situation is rapidly changing. Firstly, there is widespreadunderstanding that water alone does not create a healthy ecosystem.Secondly, many urban streams already require costly maintenance (forexample when supporting structures in box drains collapse).

Recent work by the Christchurch City Council Parks & Waterways Unitsuggests that enhancing streams by contouring the banks and plantingriparian vegetation could cost as little as 10–20% of the cost of piping thestream. Enhanced reaches retain their drainage role, but also provideecological, recreational and aesthetic assets to the city landscape. Thus,instead of being an expensive luxury, stream restoration could be anessential economy if channel maintenance work is necessary on existingurban streams.

The CCC has recently released “Waterways, wetlands and drainageguide (Ko Te Anga Whakaora mo Nga Arawai Repo)” to help bothplanners and developers with stream restoration projects. Copies areavailable from: Parks & Waterways Unit, Christchurch City Council,PO Box 237, Christchurch (ph 03 941 8872, fax 03 941 8987,waterways&wetlandsguide@ccc.govt.nz)

Teachers: thisarticle can be usedfor Biology L7 A.O.7.3a and Science:Living World L7A.O. 4. See othercurriculumconnections atwww.niwa.co.nz/pubs/wa/resources

NIWA Water & Atmosphere 10(2) 2002

13

Steamwharf Stream and Smacks Creek, and acombination of channel naturalisation andriparian planting for Papanui Stream andJacksons Creek.

In 1995, we collected information on bank andinstream habitat conditions, and invertebratecommunities at each site, just before theenhancement projects began. Five years later, thesame sites were resurveyed.

Effects of enhancementStream enhancement involved creating a morediverse range of water depths and velocities inthe streams. Bare concrete channels and a sand/gravel bed were replaced by mixtures of graveland cobbles. Different types of riparian plantingincreased bank vegetation cover at all sites.

Despite the great changes to stream habitat, wefound only small differences in invertebratecommunities before and after enhancement.Invertebrates in the enhanced streams were stilltypical of those found in urban environments –mainly snails (Potamopyrgus, Physa), worms,midges, the amphipod Paracalliope, and purse-cased caddisfly Oxyethira. Communities weregenerally species-poor, and lacked sensitivemayflies, stoneflies and most types of caddisfly.

The lack of change could simply reflect the factthat other invertebrate taxa outside the urbanwatershed are too far away to find these enhanced“islands” of good habitat (see article on pages 9–11in this issue). Another possibility (highlighted byNIWA FRST-funded research in Auckland) is thatthe stream sediment in Christchurch may containhigh levels of heavy metals such as copper, lead andzinc, as well as some hydrocarbon compounds thatcan be toxic to some invertebrates. NIWA and theCCC are currently investigating this possibility.

Changes good for fishChanges to fish communities were much morepronounced. Although we did not survey fishin the streams before they were enhanced, wecompared fish populations in enhanced reacheswith unchanged reaches in the same streams.

In the enhanced section of Steamwharf Stream,shortfinned eel densities were 1.5 times higherand inanga were found only in the enhancedsection. In Nottingham Stream shortfinned eeldensities increased by 1.6 times, andlongfinned eels were present only in theenhanced section. In Papanui Stream, wherethere had been both bank and channelenhancement, densities of bullies, shortfinnedeels and longfinned eels were 5, 6 and 7 timeshigher, respectively, than in the control section.Finally, in Jacksons Creek, no fish were foundin either control or enhanced sections of thestream. Lack of fish here reflected extensivepiping of the flow in the lower reaches,obstructing upstream migration.

Overall then, enhancement activities in thesefive Christchurch streams have resulted inbigger fish populations and greater habitatdiversity. Christchurch City Council iscommitted to the philosophy of living streams,and stream enhancement activities continue tobe a common feature in the Garden City. ■

... and at the same location 4years later, after enhancement.