I believe this to be a young Cottonmouth or Water Moccasin. Encountered on edge of field 8:45 AM with heavy dew on the ground from rains the evening before. Approximately 400 feet from the San Gabriel River where I have seen a mature Cottonmouth.
Snake about 13" long. Upon my approach it opened its mouth and moved the tip of its tail back and forth rapidly. Then after a minute of observation it headed for dense cover and disappeared.