What is device called used to measure gradient of a river?

Cant remember name but it has protracter in it, using it means looking at another point up the slope and reading off the angle.What is device called used to measure gradient of a river?
The river bed can be surveyed, and from the bed levels the gradient can be determined.What is device called used to measure gradient of a river?
You could use a level to measure the differences in height of the river bed.





Alternatively. you could use a theodolite to measure the difference in angle of points on a river bed (using a staff of a known height. but it wouldn't be very practical.