Tempted to book a river trip in a national park in the Rockies or Southwest this summer?

These landscapes are incredibly dramatic, but if you're thinking of a white-water adventure in parks such as Dinosaur National Monument, Glacier National Park, or Canyonlands National Park, sooner in the summer just might be better than later due to the possibility of a below-normal snowpack.

Unless March comes in and goes out like a lion, with some prodigious snowfalls, the runoff in the Colorado and Green River watersheds that feed Utah's iconic Green and Colorado rivers could come early...and disappear relatively fast.

Of course, those who run rivers regularly know that the quality of a river run isn't always directly proportional to runoff if you're looking for bucking wave trains. Low water flows often can amplify rapids, while high water flows can sometimes wash them out.

“It’s a funny game," says Tim Gaylord, operations director for Holiday River Expeditions that runs rafting as well as mountain bike trips in Dinosaur and Canyonlands in Utah. "Different stretches of river do better than others (in water flows). If the water isn’t real high, then a Westwater trip is going to be fantastic all year.”

As far as the Colorado River through legendary Cataract Canyon in Canyonlands National Park, depending on how the snowpack winds up later this spring, trips through the canyon "could be interesting," says Mr. Gaylord. Most folks who book trips through Cataract Canyon are hoping for high flows, he says, "but at low water there’s a challenging stretch in there, especially from the guide’s perspective.”

While the snowpacks that govern runoff through the Green and Colorado drainages currently are running about 15-20 percent below normal, farther north, along the Montana border with Canada, things are just about normal, according to Sally Thompson, a co-owner of the Glacier Raft Co. that leads trips along the Flathead River that courses down the western border of Glacier National Park.

But that near-normal snowpack doesn't mean Glacier Raft doesn't plan for the occasional low runoff years, says Ms. Thompson.

“What we’ve tried to do aftet 35 years of being in business is have lots of options," she says. "So we’ve never really been skunked when it comes to water level.”

Those options involve using smaller rafts that can negotiate any rock gardens that might surface at low flows, have customers paddle "rubber duckies" -- inflatable kayaks --, or head to different river sections that don't greatly suffer when flows are lower.

Now, low runoff could affect runs down the Yampa River through Dinosaur, as it is truly a wild river that has no upstream dam and reservoir that could be tapped to provide decent flows. If Yampa trip's are jeopardized by low flows, Holiday River can move those trips slightly west to the Green River and a run through Lodore Canyon in the northern section of Dinosaur. Not too far north of the canyon rises the Flaming Gorge Reservoir, where controlled releases keep the river floatable.

Trips through Grand Teton National Park on the Snake River also shouldn't be impacted by snowpack -- it's currently right about 100 percent of normal -- as its flows are controlled by releases from the Jackson Lake Dam.

Regardless of where you dip your paddle, you'll be surrounded by gorgeous scenery: from the heavily forested slopes that rise above the Flathead River as it streams along the border of Glacier National Park, to the soaring redrock cliffs that embrace the Colorado River as it flows through Canyonlands National Park. And then there are the Tetons -- jagged, sky-scraping walls of rock that define Wyoming's western border in Grand Teton National Park.

Some tips to keep in mind when booking your trip this year:

* Now's the time to book. Many trips are filling up with some outfitters, and you could be locked out of your preferred timespan.

* Forest fires happen, often in August, so river trips in the northern Rockies could be smoky at times.

* If you're not a gambler, look earlier in the summer to book a trip through Cataract Canyon in Canyonlands, or focus on a river where stream flows are controlled by dam releases, such as the Snake River in Grand Teton or the Green River through Dinosaur.

* Track down, and bookmark, Facebook pages maintained by rafting companies. More than a few companies offer specials to their FB followers. Also sign up for e-letters from outfitters to stay atop of river conditions, bookings, and last-minute deals.