The Nathrop gage is seasonal and probably is offline now. The next closest gage will probably be Salida, which is part of the State DWR's network. Its showing 450 cfs right now. Back a little out for other tribs coming in below Nathrop and you can expect Brown's to be running at least 400 cfs - not bad for this time of year!
here's the link to the DWR's flow website:
Colorado Division of Water Resources
have fun down there.