Anything riotous is disorderly, chaotic, and loud. Riotous things can also be abundant.
Since a riot is a disturbance where people get out of control and become violent, riotous things are also loud, disruptive, and out of hand. If everyone in class speaks at once to answer a question, that's a riotous response. Chaos is always riotous. The opposite of riotous would be calm, civilized, and peaceful. Also, you can say that lush, abundant things are riotous. If an apple tree is producing an unusually large amount of apples, it's riotous.