River is just the place for dry-and-wet warfare, in the middle there is a bridge. If you go under the bridge, you'll find a Boomshot Grenade Launcher, which can come in handy, and if you go on the bridge near the same spot the Boomshot is geographically located, you'll find a Mortar, which can prove useful if there is a sniper in a base. There is also the cycled Longshot Sniper Rifle and Torque Bow if you go up to the wall holes inside of the large bases. If you go into the small bases, the cabins, you'll find a cycle between Bolo Grenades and Ink Grenades. Right before the backyard of the cabins, are two trucks, and going between them equals obtaining either a Gorgon Burst Pistol and Boltok Pistol. The map is symmetrical, however there are, a few geometrical differences on both sides. There is also a red/orange patch of sky on one side while the sky is all dark blue on the other which can be helpful for distinguishing the two sides.
This map can be better than Security for Horde Mode if you are good with the Longshot sniper rifle. Get at least 3 players (preferably good with longshots) and get into the building. There should be a longshot that respawns, so don't take the ammo. Let your allies have a Longshot and you should be able to hold off the locusts with little to no problems. Just leave the last Locust alive each round so you and your team can get some ammo from the placed ammo crates scattered around the map. Once Maulers start to come, take their Boomshields and Roadie Run into the closest 2-story house. You can then turn to look down the stairs and plant 1-2 shields on the stairs and it keeps enemies out. Thus, it adds easy ambush situations once enemies enter the house and you and/or your teammates can snipe off the second story. Also if you are good with the Torque Bow (it cycles with the Longshot, so depending on what cycle you are on), take it. A headshot with this can be good for taking out Grinders, Boomers, and Maulers; so save it for that. Sit at the top of the house with the Longshot and wait for enemies to take cover. Headshot them when they peek up to aim, as trying to snipe them when they are Roadie Running will almost always result in a wasted bullet. Keep repeating this process, and you should beat Wave 50 easily.
At the start of each round there will generally be a rush for the Longshot/Torque Bow within the houses at the end of the roads, with the loser being at the mercy of their opponent during the pick-up-weapon animation resulting in an easy and often embarrassing kill. To ensure that your team is the one which obtains the Longshot first, it is often best to instead go for the Ink Grenade and throw it directly onto the other team's longshot spawn. This blocks access to their weapon, ensuring that your team gets the Longshot first and giving you easy kills for anyone foolish enough to try to get their own. In terms of the Torque bow on frag round, get to the Boomshot as fast as possible and, with any luck, you will get a perfect shot on the opponent trying to get the Torque bow, ensuring your team's weapon dominance for the remainder of the spawn cycle.
Because of River's layout (Resembling an "S" shaped Tetris piece), this map is well suited for Guardian. Leader's will almost certainly hole up in one of the two bases holding the Torque Bow or Longshot, since this puts them far enough away from both spawn points without having to worry about enemy fire. Try to get your team to pick up grenades whenever they spawn to give the base extra support, and stay well hidden if you aren't comfortable with either of the two long-range weapons. If you're attacking, a very quick and easy way to kill the leader is to grab Bolo grenades from the start, then head down into the stream and throw them into the enemy bases' window as the leader is holing up inside. It's also advisable to use Ink grenades in the same fashion once your teammates start an assault on their base to flush the leader out.