Did Native American reservations encourage or discourage Native American culture?

The answer to this has been different at different times
in US history.  For the most part, the reservation system has worked to encourage native
cultures, but there have been times when the opposite was
true.

In general, reservations allow native cultures to
survive more than they might without reservations.  By putting Native Americans together
in one area, the system allowed them to live together and keep their culture.  Had they
moved to cities and assimilated, their cultures would have been lost to a greater
degree.

However, there have been times when reservations
have been used to try to destroy native culture.  This was especially true in the days
of reservation boarding schools where the idea was to "kill the Indian to save the
man."  In those times, US policy was to use the reservations as an efficient way to
eradicate native culture.