Mequinensa reservoir, water

Roura M (2004) Incidence of the Mequinensa reservoir in the transport of suspended solids and in the water quality of the River Ebro. Ph D dissertation, Universitat de Barcelona... 

Like water temperature, water quality downstream from reservoirs depends on the hydrodynamic processes that take place in the reservoirs. In the case of the Ebro River, the effect of the reservoir of Mequinensa on water quality was studied by Roura [46]. According to her results, this reservoir acted as a treatment plant that improved the quality of the river water. Also, the hydrodynamics of the reservoir depends on upstream water temperature, discharge and meteorological variables. Such forcing parameters change over the years and, consequently, the water quality... 

The water temperature and thermal behaviour of the reservoirs of Mequinensa and Riba-roja have been dealt with in different studies and reports. These include water quality controls commissioned by the Ebro River Authority, the CHE [36-39], some study by the Catalan Water Authority, the ACA [42], several degree and PhD thesis at the Technical School of Civil Engineering of Barcelona [43—48] and research projects aimed to study the dispersion of the zebra mussel Dreissena polymorpha [40, 41]. However, the reservoir with the most information available regarding its thermal behaviour is the reservoir of Riba-roja as it has received an important research effort because of the invasion of zebra mussel. 

Like many other reservoirs in the temperate regions, Mequinensa is monomictic. The thermal stratification begins in spring, intensifies and attains its maximum in summer. In the autumn the water column mixes and water temperature is uniform in the vertical dimension in winter. The summer stratification is more intense close to the dam [39]. In this area, during the stratification period the surface temperatures can attain 24-27°C, while at the bottom remain around 14—16°C [36-39]. 

The reservoir of Riba-roja presents a weak stratification in summer, so that surface water temperatures are 24—27°C and 18-21°C at the bottom [36, 39], The thermocline is situated between 14 and 21 m deep [38, 39], During this time of the year, the water entering the reservoir of Riba-roja from the reservoir of Mequinensa is colder than the water coming from the Segre River. In consequence, the denser Ebro River water sinks and flows along the bottom of the reservoir while the Segre River water floats and flows along the surface [40, 41], The deep water can be anoxic [36, 38, 39], In winter, the water temperature is uniform in the water column and the minimum values are around 11°C [38],... 

Ward [49], Barnes and Minshall [5] and Ward [6] describe the typical effects of deep-release reservoirs on downstream water temperature higher temperatures in winter, lower temperatures in summer, reduced daily and annual thermal amplitude and displaced annual maximum and minimum water temperature. These effects have been observed when studying the reservoir of Mequinensa. While just upstream from the reservoir, at Escatron, the annual thermal amplitude is about 19°C, downstream from the reservoir it is reduced to about 15°C and the annual maxima occur about 20 days later. Also, the effect on water temperature varies along the year on a cyclical basis. In November, the mean monthly water temperature at Escatron can be as much as 6°C higher than that of the water exiting the reservoir. On the other hand, it can be as much as 4°C lower in May. 

In the reservoir of Riba-roja, water from the Segre River somewhat mitigates the effect of the reservoir of Mequinensa, although the impact of reservoirs downstream of the system of reservoirs is still remarkable, as demonstrated by... 

Fig. 7 Water temperature variation caused by the system of reservoirs of Mequinensa, Riba-roja and Flix and by the nuclear power plant of Asco...

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