Simply You can correlate to assess the effects of flow on vegetation cover .

I might try looking at the rainfall in periods from the last vegetation assessment to the current vegetation count. I could see that an average rainfall might be important. Variance and periods of flood or drought might also be important. Before analysis, I would try graphing the data. Relationships might not be linear. I would also expect thresholds. Flooding for 2 days has little effect, but 3 weeks causes more extensive damage.

Temperature might influence vegetation growth as much or more than rainfall.