One of the tenets of journalism is that there are no stupid questions, so I guess I’m not embarrassed to ask this: Why hasn’t the Merrimack River run out of water?

Think about it. We’ve had almost no rain for two months – this summer was officially the driest on record in New Hampshire – and it only takes a week for a drop of water to travel from Franconia Notch down to Concord. Doesn’t it seem like all the drops should have flowed past us by now?

Go north on I-93 and check out the Pemigewasset River, which flows into the Merrimack; it’s almost down to rocks and puddles, but the Merrimack in Concord is still flowing along.

I took my puzzlement to Ted Diers, assistant director of the water division at the state Department Environmental Services. His answer: “Base flow.” Turns out, it’s a hydrogeological way of saying that it oozes out of the soil.

When rain falls on the ground, some of it gets taken up by plants and some of it seeps down into the ground. Over time — days, weeks, months or years depending on geology — gravity pulls that water down and sideways through the ground, following contours caused by rocks or different soils, until it comes to a place where there’s no more dirt and it falls out. That is the river bed. “Rivers are generally the low spot in an area,” Diers reminded me.

Even if no rain has fallen for weeks, rain that fell months or years ago anywhere in the watershed — the area where all water flows into a given river — keeps entering after a long trip underground. That is the base flow: “What each watershed is contributing on a normal everyday basis into the river, as part of normal drainage.”

Underlying geology explains why the Merrimack River is still full but the Pemigewasset looks almost drained.

“In rivers up north, there’s not a lot of stuff above the rocks, not a lot of fill, sand, not a lot of soil,” Diers said. “The Ammonoosuc, the Pemi, they’re really bony so there’s less to hold the water.” With less soil, there’s less water oozing into the river beds.

The Merrimack River watershed covers 5,000 square miles. While only about a quarter is upstream of Concord, that’s still a lot of land to have gathered water from the sky. “It’s a diverse watershed, from the Souhegan up to the Pemi. You can drive three hours and be in that same watershed. It covers a lot of different geologies,” Diers said.

This means, incidentally, that the water we see can be of very different ages. One drop might have fallen in rain near the river bank and made it to Concord in a few days, while the drop next to it might have fallen on a hill behind Plymouth and taken years to get to the Pemigewasset and then the Merrimack.

The speed that water travels through soil is very hard to know.

“You can’t know long it takes rain to get to the river,” Diers said. “It depends on how low the water is, what the soils, what the slopes are. There’s a million different variables.”

It is possible to determine the age and history of any given bit of water through isotope hydrology, which is an entire subsection of the field. But that’s a bit outside my pay grade.

The existence of dams across the Merrimack, from Sewalls Falls down to Lawrence, Mass., also help hold back water although none are designed to create actual reservoirs, so their effect is less than you might think.

Obviously the ability for a river to keep filling itself with groundwater is limited. As they are seeing out west and in other parched areas around the world, at some point there won’t be any water left in the ground unless it has been recharged by rain in the watershed.

That’s not a concern here in New Hampshire, or at least it hasn’t been. The way climate change is scrambling weather patterns, though, we shouldn’t be too complacent.