"If the resources of forest and water power were more fully developed, the economy would not have been so dependent on import."
The sentence is almost correct. You need to use 'imports' instead of 'import'. The plural form is required because there is more than one product or import that we would be dependent upon.
So the sentence should be:
"If the resources of forest and water power were more fully developed , the economy would not have been so dependent on imports."
One other suggestion. Your 'resource of forest' wording is correct, but it is far more common to say, 'forest resource'.
So, what you wrote is fine, but myself, I would write:
"If forest resources and water power were more fully developed , the economy would not have been so dependent on imports."