I just started watching Season 1

I'm about at episode 22 or something.

I used to watch the show when it was on TV but I didn't start watching until around 2000-2001. Clooney, Carol (don't know the actresses name), Susan Lewis, William Macy, Michael Ironside and Green's wife were not parts of the show then.

Carter, Green, Benton, Chen and a few others were still around.

I know I'll find out if I keep watching but how does Clooney (Ross) leave the show? Green is not married and dating the British doctor Elizabeth Corday. How does their marriage dissolve? How, why does Lewis leave as well?