How to determine when a river makes a 90 degree turn.

Discussion created by kari66 on Nov 29, 2012
I am new to Hydro. I am trying to find a way to select or highlight a river when it makes a 90 degree turn. For example if the Nile
flows S.-N. its origin is at 180 degrees and its end point is roughly 1 degree. I am searching for a way to show rivers that make a 90
degree turn or an "L" shape. I am ok with a river's azimuth being +-10 or 15 degrees from straight.

What tool can I use to accomplish this?

I have downloaded some files from the NHD from USGS.