During 1967, surveys of five tidal cycles (June, July, August, September and November) were conducted in the Vellar Estuary. The survey was so designed that it covered almost the whole summer and rainy seasons. Measurements of salinity and temperature were made in 17 fixed stations in the estuary which were visited in sequence. The data collected were analysed to know the distribution of salinity and also the mixing pattern in the estuary. From the results the following inference was made.

During June-July, when there was very little fresh water discharge in the river, the estuary remained predominantly marine, the mixing being nearly complete and without any marked vertical gradient in salinity. However during the flood tide there was an intrusion of a saline wedge along the bottom. The penetration of the saline wedge towards the head of the estuary extended to about 3.5 km along the bottom. Even during this period, there was a moderate horizontal and vertical salinity gradient in the upper part of the estuary.

During August, due to a small amount of river discharge, the pattern was changed slightly. The mixing of the two waters was rather incomplete, throughout the estuary, due to which, a moderate vertical salinity gradient was established in the lower part of the estuary, while the same was more pronounced in the head of the estuary. The penetration of the saline wedge was much restricted and extended only to about 2 km from the mouth of the river. The more or less low salinity water in the surface extended down to a depth of about 0.5 to 0.7 metre, below which there was a halocline.

In September and November, the river discharge was considerably large and as a result, the vertical as well as horizontal halocline was well established, in the whole estuary. The low salinity surface water was further pushed downward to a level of about 1 metre in November and the saline wedge became less pronounced.

The isohalines were rather smooth and less steeply inclined during August, September and November than what they were during June-July.