I find Colvin confusing -- Help?

Basically, I’m fairly confused by Colvin’s argument. I understand his basic thesis: deterioration of the Adirondack watershed would compromise the Erie Canal, leading to certain economic disaster. This passage makes his thesis fairly clear: “Without a steady, constant supply of water from these streams of the wilderness, our canals [specifically the Erie] would be dry, and a great portion of the grain and other produce of the western part of the state would be unable to find cheap transportation to the markets of the Hudson river valley.”

But what I fail to understand is how, exactly, the Adirondack watershed would deteriorate or dry up, as per Colvin’s hypothesis. Colvin’s semi-scientific explanations are extremely confusing to me. I've tried to find a few of the problematic passages, so that one of you could share your thoughts on Colvin's arguments and hopefully clarify a few things for me. 

Perplexing Passage #1

“It is now generally conceded that forests do not increase the amount of annual rain-fall. Their influence is to cause a distribution of the rain in frequent showers at short intervals throughout the year, while their absence induces droughts, followed by sudden and tremendous storms which are the origin of disastrous floods” (108). 

What I think this passage is trying to say: I’m surmising here that Colvin’s contention was not that forests increase rainfall, but that they help evenly distribute rainfall and minimize large storms which leading to flooding.

What I’m confused about: Is there any scientific evidence supporting this? Do forests actually have a correlation with weather patterns?

Another passage cites a meteorological experiment based in Africa, which showed that “the rain fell plenteously upon the forest, while the neighboring plains received no showers.” Colvin used this as evidence of the forest causing an increase in rainfall. However, this evidence seems pretty insufficient to me, and we’re still left with the question of whether this pattern represents “causation” or mere correlation. For example, geographical areas that receive less rain are less likely to sustain forests. It might not be the forests that cause rain, but rather the rain that causes forest growth. Did anyone else pick up on other evidence that supported Colvin’s thesis?

Perplexing Passage #2:

“there are long periods during which rain does not fall; the land is parched by the sun’s fervent heat; evaporation proceeds rapidly; the air, though clear, is saturated with invisible moisture, heavily charged with electricity, which finds the moist upper air a better conductor than the heated, dry, repellant surface of the earth. The drought continues; nature is silently becoming exasperated, for there are now (in this case) no broad, cool forests, nor cold, wooded mountain sides to condense the vapor into cloud. The only chance for the relief of the atmosphere from its burden of moisture is the advent of some cold north wind. it comes at length , and the region of the air is convulsed. Black, gloomy clouds rapidly gather and the suspended vaporous ocean overhead drops suddenly one grand deluge” (109).

What I think this passage is trying to say: If precipitation does not occur often, then it gathers in dark, gloomy clouds, and comes down all at once leading to a gigantic storm and flood.

What I’m confused about: does this claim have an scientific substance? Do any of Colvin’s claims have any scientific substance? Does a forest preserve protect the watershed’s transporting ability at all?

In short, I’m wondering if anyone else understands Colvin’s scientific grounding. And if so, how much of his argument is obsolete and disproven? It seems that although his desire was to protect the Adirondack watershed for transportation purposes, none of his contentions had any actual scientific backing. While I’m thankful that his First Annual Report led to the formation of the Adirondack Park and Forest Preserve, I’m wondering if—at this point—his reasoning had any actual scientific merit to it (and that the positive ecological effect of the forest preserve was not just happenstance).