To find the water level at the end of the summer, we need to calculate the total drop of the river below sea level.
The initial drop of the river is 1 1/2 meters below sea level.
The additional drop during the summer is 2 1/3 meters below sea level.
To calculate the total drop, we add the initial drop and the additional drop: 1 1/2 meters + 2 1/3 meters = 3 meters + 2 meters = 5 meters.
Therefore, the water level at the end of the summer is 5 meters below sea level. Answer: \boxed{5}.