RMZ-Materials and Geoenvironment:Vol. 51, No. 3, pp ……, 2004

The Water Level Fluctuations as an Indicator of Tufa Barriers Growth Dynamics in the PlitviceLakes

Gordana Zwicker1, Josip Rubinic2

1PublicInstitutionPlitviceLakesNational Park, ConservationResearchCenter "Ivo Pevalek", 53231 Plitvicka jezera, Croatia, e-mail:

2 Faculty of Civil Engineering, Department for Hydrotechnic and Geotechnic, University of Rijeka, V.Cara Emina 5, 51000 Rijeka, Croatia, e-mail:

Abstract: The dynamical processes of tufa formation define the morphological and other

characteristics of PlitviceLakes system. As an indicator of tufa barriers growth

dynamics in the PlitviceLakes is used hydrological methods – analysis of water

discharge and water level fluctuations.

Key words: PlitviceLakes, tufa formation, hydrological methods

Introduction

The PlitviceLakes are unique nature phenomenon of Dinaric karst that was introduced in to UNESCO's World heritage list as late as 1979. The 16 lakes separated with tufa barriers are devided in two groups with total volume around 400.000 m3. Origin of the lakes and also their evolution are connected with intensive tufa formation processes. These processes are studied and documented by numerous scientists (the short review was given in the work of Božičević & Stilinović (1998). It has been found that biogenic factors play active role in tufa formation. The results of tufa dating with 14C method showed that active barriers were formed during the last 6.000 – 7.000 years (Srdoč & al., 1985). The age of paleo tufa located on the old river banks out of water course was determinated with 230Th/234U method. The results showed that these barriers are between 90.000 and 130.000 years old (interglacial period Riss-Würm). There are also detected some tufa samples which are between 250.000-300.000 years old from earlier Mindel-Riss interglacial period (Obelić & al., 2000).

The biggest, and also the deepest lakes are Prošćansko and KozjakLake. The depth of KozjakLake is 45 m, with surface of 0,83 km2. In this lake was found the biggest flooded tufa barrier with peak around 5-6 m under the water surface. This barrier is around 22 m high on upstream side and even 37 m high on downstream side. Faster tufa growth on the profile of existing barrier at the end of KozjakLake caused upstream water level rising, flooding of barrier and joining of two before divided lakes. According to the research results of Srdoč & al. (1985) based on 14C analyses of spruce trunk taken out from Kozjak Lake with dated age of 710 years, it has been determined that average rate of water level rising was 1,35 cm per year during that period. It is 17 times faster than sedimentation of calcite mud at the lake bottom during the same period.

Since there was no assessment of lake level rising dynamics or tufa barriers growth, especially in recent years, in this paper are represented analysis of available hydrological data. Analyzed hydrological stations located on the biggest lakes (Kozjak and ProšćanskoLake) had long-term discontinuity in their work due to war operations in this area during the early 1990's. Therefore data collected from the period 1952-1990 were taken into consideration.

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Results and discussion

Analyzed fluctuation trends of minimal and average annual water levels in Prošćansko and KozjakLake show evident increasing trend (Figure 1). Minimal and average annual discharge on station Kozjak were analyzed to explore if increasing trend is result of changes in discharge regime. The discharge was measured and monitored only on hydrological station Kozjak. It has been determinated that movements of discharge trends are in the opposition to fluctuation trends of characteristic water levels. In other words, through the system of PlitviceLakes water discharge decrease is present, which is characteristic for the whole area of Dinaric karst, but also wider. Calculated trend of annual average water discharge decrease is 0,041 m3s-1/year (1,15%), and for the minimal discharge is 0,0042 m3s-1/year (0,4%).

Figure 1: Overview of fluctuations of average (av) and minimal (min) annual water level on Prošćansko and Kozjak lakes, and related trends.

To estimate the influence of observed water discharge decrease trends on the Kozjak Lake (for which discharges were available) on water level changes, except mentioned analysis, data in consumption relations obtained on station Kozjak-most were used. It is calculated that discharge decrease trends through 39 year period resulted in decreasing of annual average discharge for 0,165 m3s-1. If we place this data of station Kozjak for the last available year (1990) in consumption relation, average annual discharge decreasing trend, in relation with correspondent water level, caused decreasing of water level for around 10 cm on the basis of annual average data, or around 2 cm on the basis of average minimal annual values. Because of faster elevation of tufa barriers, this component of water level decreasing in the lakes wasn't noticed, but on the contrary, overall water level increasing was observed. Therefore it is necessary to add this data to data obtained according to the analysis of water level increasing trends on the KozjakLake throughout 39 year period (on average 0,30 cm/year for mean and 0,53 cm/year for minimal annual water level). It was obtained for both analyzed series that total real increasing of water level in KozjakLake caused with tufa barrier growth as 22 cm, or on average 0,56 cm/year. It is calculated that total increase was 12+10 cm for annual average water levels or 20+2 cm for minimal annual levels.

The water discharges data on ProšćanskoLake weren't available so the overall estimation of water level changes could not be done, as it was for KozjakLake. Without that component, only using analyses of average annual water levels variation it was calculated increase of water level for 44 cm or even 55 cm according to the analyses of minimal annual values. This indicate that tufa barrier growth in ProšćanskoLake is three times higher, or 1,5 cm/year. It is very close to the before estimated value of 1,35 cm/year (Srdoč & al, 1985) but for the KozjakLake. At the end of 90's water level measurements on both lakes were renewed. The new results show even more emphasized increasing of water level (possibly because of increased vegetational overgrowth of tufa spillover). However, this series could not be included in this paper because of small amount of data.

Conclusion

This paper displayed that with relatively simple hydrological analyses of available hydrological data it can be obtained very valuable results of recent tufa growth dynamics on PlitviceLakes. It is determined that tufa growth is 0,56 cm/year on KozjakLake whereas on ProšćanskoLake is even three times higher,

References

Srdoč, D., Horvatinčić, N., Obelić, B., Krajcar, I. & Sliepčević (1985): Calcite deposition processes in karstwaters with special emphasis on the Plitvice lakes, Yugoslavia. Carsus Iugoslaviae; Vol. 1, No. 4-6, pp. 101-213, Zagreb.

Stilinović, B. & Božićević, S. (1998): The PlitviceLakes. European Water Management; Vol. 1, No. 1, pp. 15-24.

Obelić, B., Horvatinčić, N., Krajcar-Bronić, I. (2000): Fizikalno-kemijska i izotopna istraživanja vode i sedre u Nacionalnom parku Plitvička jezera. Procc. 50 godina Nacionalnog parka Plitvička jezera, HAZU i Društvo za zaštitu Plitvičkih jezera, pp. 25-36, Zagreb.