Three young adults encounter large aggressive biped while fishing the San Joaquin River witnessed in Fresno County, California

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My story occured around 1991 on the San Joaquin River in what would be the northwestern part of town. Actually in Fresno. The area is mostly farm land and some sparse woods and vineyards. nothing like foresty though. My self, my cousin and a friend had decided to go late night fishing at a little area on the river where the river sort of pools. Their is a bridge there. It was late and we were sitting there and fishing when we heard something making noise and looked back and saw a coyote running across the bridge heading north. The coyote kept looking back as it ran as if it was frightened.The coyote looked at us but it was clearly focused on something behind it and it was scared. We thought at that time that that was odd. We then heard what we thought was a splash as if someone had dove into the water behind us. It was dark so we just glanced back but of course saw nothing. We probably sat for about 10 minutes or so when suddenly there was a huge goosh and splash about 25 yards away in...