General manager Brian MacLellan announced the moves Monday. Since Reirden was promoted from associate coach to fill Barry Trotz's role, Arniel and Cashman have replaced him and outgoing assistant Lane Lambert behind the bench for the defending Stanley Cup champions.
The 55-year-old Arniel spent the past five seasons as associate coach for the New York Rangers under Alain Vigneault. Arniel previously spent parts of two seasons as coach of the Columbus Blue Jackets.
The 35-year-old Cashman was an assistant with the American Hockey League's Hershey Bears the past two years. Like Reirden, Cashman played defense and could be a fit to coach the Capitals' defensemen.