I'm about to invest in a few head of cattle (8) and so I've been debating on the best way to keep them watered. One option is an elaborate pump system ran out to the pasture, or the other option is to simply dig a small stock tank. (According to FarmLogs it would be 1/8 acre or less in size.)

The area highlighted in yellow in the picture below is a depressed section of my pasture (about 3' lower than the rest) which already collects water about a foot deep when it rains heavily or the creek (in blue) overflows.

My initial questions are:
1. Would this be a good place to dig since water already collects there? (It's solid clay.)
2. Are there any concerns with the creek being that close? (About 40 feet away)
3. If any of you own cattle, would that size tank suffice for 8 head?