OPTICAL PROPERTIES OF LAKB COLERIDGE: THE IMPACT OF' IIYDROELECTRIC PO\ryER DEVELOPMENT I OPTICAL PROPERTIES OF LAKE COLERIDGE: POI/üER DEVELOPMENT for Electricorp) ROBERT J DAVTES-COLLEY !ìIATER QUAI,ITY CENTRE D.s .r.R. P O BOX 11-115 HA}IILTON, NZ JOHN K FEN}IICK WATER RESOURCES SURVEY D.S.I.R. P O BOX 13-695 CHRISTCHURCH, NZ Running Head,Iine: Lake Coleridge, Optical Properties TABLE OF CONTENTS Page EXECUTIVE SUMMARY TECHNICAL SUMMARY INTRODUCTION STUDY AREA METHODS 9 RESULTS 11 Lake Water Colour 11 Visual CIaritY 12 r,ight Attenuatj-on and. Reflectance 13 Water Composition and Optical Character 15 Absorption and Scattering of r,ight 16 Inflow of Suspended Sediments 18 DISCUSSION 19 Effects of the Diversion Waters on Visual Clarity 19 Regression Models 20 optical Model 22 Relation of In-Lake Suspended Sediments to Sediment Inflow 24 CONCLUSIONS 25 27 RECO¡4MENDATIONS FOR FUTURE WORK EXECUTIVE SUMMARY Lake Coleridge is generally a visually attractive, clear, water body the optical character of which largely reflects the i-nfluence of fresh water rather than that of water constituents. The greenish-blue hue is essentially that of pure water, but the colour is brighter and greyer than optically pure water because of the presence of fine suspended inorganic sediments. These sediments appear to be derived mainly from diverted \,raters of the Harper and V{ilberforce Rivers although there is only a weak correlation of in-lake concentrations with diversion sediment loads. a significant decrease in visual clarity of the lake as measured by Secchi disk depth (which fell from 13.4 to 8.6 m) has occurred since the oakden Canal was comnissioned. Relationships developed in this study, between Secchi depth and other optical measures¡ suggest that this decrease in visual clarity has been acconpanied by a drop of lesser magnitude in light penetration for plant photosynthesis and an increase in brightness of the water colour. A relationship between inlake suspended matter and inflowing suspended Loads in the diversion \,¡ater \¡¡as not established because of incompleÈe sediment inflow records, no data on sediment grain sizes involved, and the complicated hydro and sedirnent dynamics of the lake. A programme of monitoring for the life of the water right, based on Secchi disk observations at a central Iake site, is recommended in order to track any long term trend in clarity and to provide a database against which to measure the impact of any future changes in lake management. If a change in lake management policy is being contemplated then a more detailed study to enable the optical properÈies to be modelled, from the quantity and type of inflow sedimentsr mâY be required. TECHNICAL SUMMARY A one year study of the optical- properties of Lake Coleridge has been carried out to determine the effects of inflows of turbid waters in the Wil-berforce and Harper Rivers which augment hydro-electric po\¡rer generation at the Coleridge Power Station. The objectives of the study were (a) to determine the factors controlling the optical character of the Iake, including visual- clarity, Iight climate and colour, (b) to assess the ecological and aesthetic impacts (if any) of the dj-version waters via changed optj-cal properties and (c) to recommend a programme of long term monitoring f or the l-ake. The study has shown that the blue colour of the water in Lake Coleridge largely reflects the influence of v/ater itself, rather than that of additionaf constituents, on the optical properties. However, at certain tirnes the lake water colour appears somewhat greyer and appreciably brighter than pure water. This depends on the concentration of inorganic suspended sediments from the d.iversion waters. These inorganic sediments dominate the attenuation of image-forming light in Lake Coleridge, mainly by the process of scattering, and thus contribute strongly to reduction in visual clarity as measured by Secchi depth. Phytoplankton biomass, in this oligotrophic lake, is low and has littl-e influence on the overall optical character. Comparison of mid-lake Secchi depth measurements made in this study with historical- data suggests that visual clarity has been significantly reduced from about 13.4 m to 8.6 m since Èhe Oakden canal was commissioned. The visual clarity is still "high" r but the reduction may represent an impact on aesthetic quality of the lake and, to a lesser extent, on the ecology, by reducing the sighting range of fish for example. However, it is difficult to assess the significance of this reduction in clarity in terms of recreational use of the Iake in the absence of guidelines or criteria for aesthetic quality. It is thought likeJ-y that this reduction in clarity has probably stabilised at a ner¡r steady state, but this is not known with certainty. The penetration of diffuse ambient light into lake Coleridge is "high" (cornparable to that in Lake Taupo) with 1Z of light reaching about 30 m 5 depth on average. This euphotic depth may be less compared with pre-diversion times, although to a lesser extent than the visual clarity. This is because the diversion \¡tater sediments scatter light. strongly but are only weakly lighÈ absorbing. OnIy s1ight impacÈs on lake ecology are expected from this change in light climate, namely a small reduction in benthic plant cover and thus in available habitat for aquatic fauna. Simple models have been developed, based on linear regression and on established optical theorY. These models relate the oPtical, characteristics of Lake Coleridge to in-lake total suspended sediment (TSS) concentrations. However, it was not possible to predict in-lake TSS from mass flow of sediment in the diversion waters (and other inflows) because of incomplete mass infl-ow data, and the complexity of inflow/Iake hydraulics, and the wide dispersion of grain sizes (and hence of faII velocities and of scattering per unit mass of solids). Thus we cannot, at present, model the effects on the optical character of Lake Coleridge of changed diversion water Ioadings. Long term monitoring, at a low level of activity, is desirable to detected any long term trends in clarity (e.g. from continued shore erosion) and Èo provide data for assessment of the impact of any future changes in management of the lake. It is recommended that Secchi depths be measured at monthly intervals at a mid-take site, and thaÈ water samples be obtained for analysis of suspended sediments and turbidity (the latter as a quick and simple check on data consistency). The Secchi, turbidity and TSS data should be plotted immediately they are available, to provide a time series which will facilitate examination of trends in optical quality of Lake Coleridge. If changes in Lake management are being conÈemplated Èhen a more detailed study to develop a model of Iake optical properties, from Èhe quantity and quality of inflow sedimentsr f,âY be worthwhile. INTRODUCTION Lake Coleridge, in the Canterbury high country (Fig 1), is a rnajor fishery and a tourist asset of regional, and perhaps, national importance. The lake also provides water storage for hydro-electric power generation at the Coleridge Power Station, situated near the southern shore. The station began operating in 1914 using water flowing naturall-y into the lake. Ho\^/ever, increasing electricity demand has resulted in the need to augment the inflows (to increase ttre generating potential) with waters from the glaciated Harper and !{i}berforce catchments in 1921 and 1977, respectively. An environmental impact report was prepared (in 1975) to cover the final-, Wilberforce, part of the development (Jo\^/ett 1984). one of three major concerns identified in submissi-ons on this report was that the discharge of Wilberforce waters, including glacial neltvrater laden with high suspended matter concentrations, \^roul-d reduce the general clarity of Lake Coleridge (Jowett 1984). A further concern has been with the potential for increased phytoplankton production in response to the increased nutrient load in the diverted water (t',titcfrett 1984) . Suspended sofids mainly affect receiving waters by attenuating light (principally through scaÈtering - Kirk 1985). This results in reduction of clarity and. changes in colour. The change in colour is mainly attributable to an increase in the ratio of Iight scattering to tight absorption. This causes an increase in brightness of the water, but a d,ecrease in colour saturation (ie, increased "gtreyness" of the colour) also occurs. Hue (eg, blue, green, yellow) may also change if the mineral sediment is associated v/ith light-absorbing organic materj-al. Reduction in waÈer clarity could irnpact the lakets ecology and recreational value in two main ways. Firstly, reduced visual clarity (traditionally measured by Secchi disk depth) would reduce the visual range of aquatic organisms and of recreational users of the lake. Secondly, the penetration of diffuse photosynthetically available radiation (PAR) which determines the depth to which plants can grohT in the water (euphotj-c depth) rnight be reduced with consequent impact on primary production and on the depth range of plants. These two aspects of v/ater clarity reflect rather different optical processes. Thus, there is no analyticat relationship between 7 visual range in water and penetration of diffuse light in water, although a broad correlation is often noted. To date it has not been demonstrated that any significant reduction in water clarity (or change in colour) has occurred following diversion of llilberforce vrater to the lake. Hovrever, Jowett (1984) using a regression model he developed for Secchi depth (based on residence time, mean depÈh and percentage glaciation of 24 large New Zealand lakes , t2 = O.77) predicted that the Secchi disk clarity in the centre of Lake Coleridge would stabilise at about 1 metre less than pre-diversion levels.