Do a search on freshwater dips on this message board. They are stressful, but most healthy clams will survive them. I have had good experience by dipping my clams for 15 minutes in freshwater that is the same temp and has been pH adjusted to 8.2 (using the product "Proper pH 8.2"). Others on this board recommend longer dips - i.e. up to 30 minutes.
Be careful - healthy clams seem to take dips very well. However clams that are unhealthy and/or have pinched mantles don't seem to fare as well. Clams HAVE died as a direct/indirect result of FW dips, so be warned.