Google Advanced Search
Search for your topic keywords using the top search box, and in the "site or domain" box limit to .gov
Also consider using "data set" or "database" in your search. For example:
Mississippi River water quality data sets (limit to .gov sites) = AWESOME results, including a USGS sediment-contaminant database
Remember that .com sites are usually trying to sell something (or make money off you viewing the ads!) and .edus can be either professors' or students' sites.