Aa 18-year-old is facing a criminal assault charge after he was accused of pushing an 8-year-old boy off a slide at a suburban Minneapolis water park.

Authorities say the boy was tossed over the edge, falling 10 metres onto the concrete below.

Roman Adams and the 8-year-old were waiting in line to ride down the waterslide, a criminal complaint stated.

Once they reached the top platform, Adams picked up the boy and pushed him over the railing, telling police he threw the child because he was frustrated about the line taking too long.

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The 8-year-old was conscious and breathing when officers arrived.

He reportedly suffered a broken femur, fractures to both feet, and shattered bones in the shoulder.

He was taken to hospital where he is in a stable condition.

Police believe the 18-year-old may be developmentally delayed, but "knew what he did was wrong", Apple Valley Captain Nick Francis told news station KTSP.

Adams is charged with felony third-degree assault. He has been released from jail ahead of a court appearance on September 17.