It will strrongly depend on the limnological characteristics of this river, such as the mean flow and natural tamperature, as well as the climatologic chraracteristics of the region it crosses, in order to couple with the characteristics of the discharge flow. I suppose you have already considered these informations. I suggest you, if possible to make some experiments with sentinel species of this river, in order to test their physiological responses regarding the temperature in question. In the literature there are a lot of protocols, since the behavioural to the more sensitive biochemicals, where some importat cell proteins are tested.

It also depends on the seasonal or behavioral or life-stage-dependent habitat requirements of the species of interest. These are dynamic factors and the 'best' thermal regime could be just as dynamic. What you could do is to use a good numerical model to predict what the 'natural' conditions would be in the absence of the discharge and use that to 'index' the variation of the discharge, staying, of course, within the limits of habitat or tolerance extremes. Or you could use a similar, nearby river as an 'index' or 'reference' system to accomplish the same goal. Good luck.

The higher temperature may be allowed when the natural (without effluent) temperature of river is the lowest,say during dead if night & lower temperature of 24 degree celsius during day time.At the same time,thermal tests with especially the delicate species need to be carried out,in order to assess the optimum temperature they can with stand.The amount of river flow & possibility of cold wind having a tendency to cool waters also need to be taken into consideration.An alternate method would be to pass the thermal discharge through metal pipe meanderings,suspended in air to dissipate heat in air,in order to cool the discharge to tolerable levels.Similar action was taken in case of thermal effluent discharge in case of a soda ash plant by first pouring the effluent into immense coastal ponds,cooling the effluent & then releasing into coastal marine waters.

If possible, the natural temperature regime (diurnal and seasonal) should be noted before any hot water discharge; and be used as a guideline. Unfortunately It usually is time consuming and difficult to adequately note the effect of temperature change (increase in this case) on organisms, unless there have been long term (i year or more in duration) studies in the same area. For instance, a temp increase may not outright kill an organism but affect its breeding success or juvenile development or diminish its prey base, that over time may constitute a stressor. If the natural temperature regime is known, one can try to maintain that variation, and try ways to reduce heat before entering the river, as mentioned by Trivedi et al.

The temperature of water temperature within 100 meter distance from discharge point should not exceed 3 degree centigrade.

There are several things regarding the temperature profile of an aquatic system, high fluctuation may leads to mild to high alterations in that particular system; some of the mobile organisms may avoid the scenario but there are several non-motile organisms as well as plant species which may be in the threat of temperature intolerance; the temperature profile may vary depending on the atmospheric condition, before let into the system know the level of temperature @ where the outlet enters the system. it may vary slightly not in a medium or maximal level.

good luck; have a nice day

Even with the ecological knowledge gained from the suggestions made by other respondents, the main practical limitation could be a practical one. That is, what sort of water release infrastructure you have. For example, is it a single or multipe off-take and/or there is capacity to vary the temperature. If the answer is yes, then perhaps mimicing the natural/ambient temperature would be a start.

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