Linear Modeling the best alternative

I have a data set of mean velocity, discharge, and mean depth.  I need
to find out which model best fits them out of log linear, linear, some
other kind of model...  Using excel I have found that linear is not
that bad and log10(discharge) vs. the other two variables (I am trying
to predict velocity and depth from discharge) is not that bad either.
How do I test and see which one of these models is better...  better
R-squared...  I know this is a stats question and not particularly an
R question, but I will use R for the models vetting process.
any ideas would be greatly appreciated,

Stephen Sefick
Research Scientist
Southeastern Natural Sciences Academy

Let's not spend our time and resources thinking about things that are
so little or so large that all they really do for us is puff us up and
make us feel like gods. We are mammals, and have not exhausted the
annoying little problems of being mammals.

	-K. Mullis