New Hampshire waterfalls: two Mt. Washington Valley guides on Conway Granite.

The honest answer is that the title is disputed. Arethusa Falls in Crawford Notch State Park is most commonly cited as the tallest single-drop waterfall in New Hampshire and is the figure used by the NH State Parks listing and most modern guidebooks. The height itself is contested: Visit New Hampshire and Visit White Mountains list 160 feet, the Wikipedia article and current signage list 140 feet, and a 2017 Woodlands and Waters field write-up estimated about 176 feet. Waterfalls Guide treats 140 ft as the conservative authoritative number. Other candidates including Wilkinson Falls and Beecher Cascades contest the ranking by total drop, but Beecher is multi-tier rather than a single fall and Wilkinson is harder to access. Arethusa wins on uninterrupted single-drop height; it loses on volume to Glen Ellis Falls in Pinkham Notch.

The White Mountains hold dozens of named waterfalls across the White Mountain National Forest and several state parks. The two Waterfalls Guide currently publishes in full, Arethusa Falls in Crawford Notch and Diana's Baths in Bartlett, are the headline cluster on the US-302 and West Side Road corridor in the Mt. Washington Valley. Other well-known White Mountain falls include Glen Ellis Falls and Crystal Cascade in Pinkham Notch on NH-16, Sabbaday Falls and the Lower Falls Scenic Area on the Kancamagus Highway (NH-112), Avalanche Falls inside the Flume Gorge at Franconia Notch State Park, and Beecher Cascades north of the Arethusa trailhead in Crawford Notch. The USFS White Mountain National Forest page is the standard reference for falls not yet published here.

Two windows. Late April through mid-May is the snowmelt peak, when Bemis Brook and Lucy Brook drain hard off the Presidential Range and Arethusa reads as a full white sheet rather than a thin horsetail. The third week of September into the first two weeks of October is the fall foliage window, when the Mt. Washington Valley hardwood color fills the Crawford Notch corridor and the Lucy Brook canopy. Summer (late June through August) is the swim window at Diana's Baths with pool water in the low 60s F. Winter trips require microspikes or full traction on both trails, and the Arethusa Falls trailhead lot is unplowed.

Most are free. Crawford Notch State Park charges no entry fee for the Arethusa Falls Trailhead, and most White Mountain National Forest waterfalls are free at the trailhead. Diana's Baths is the practical exception: the USFS day-use lot on West Side Road charges $5 per vehicle (cash or America the Beautiful interagency pass). The Flume Gorge at Franconia Notch State Park, where Avalanche Falls sits, charges a paid admission. Several New Hampshire state-park sites also charge a per-person day-use fee in summer; check the NH State Parks page before you go.

Some, but not most. Diana's Baths on Lucy Brook is the most famous family swim hole in the Mt. Washington Valley: the lower pools are knee- to thigh-deep with a waist- to shoulder-deep tub at the foot of the upper cascade. Water stays cold (roughly 60 to 65 F in midsummer) and there are no lifeguards. Arethusa Falls has no real swimming hole; the plunge pool is shallow, the granite is slick, and wading is informally tolerated only on hot days. For warm-water swimming with lifeguards the standard alternatives are Echo Lake State Park near North Conway, the Saco River pools below Bartlett, and Jackson Falls on the Wildcat Brook side.

The southern White Mountains run on the Conway Granite of the White Mountain Magma Series, a pink-to-buff intrusive rock emplaced into the older New England bedrock roughly 180 to 165 million years ago during Jurassic-age volcanic activity. The same magma series built the ring-dike complexes that today form Mount Washington and the Presidential Range, the Ossipee Mountains to the south, and the Crawford Notch cliffs that hold Arethusa Falls. Pleistocene continental ice overrode the region until about 12,000 years ago and overdeepened the main valleys, leaving hanging tributaries like Bemis Brook to drop off the cliffs as waterfalls. At Diana's Baths, the same granite is the canvas for plunge-pool potholes: small cobbles spun in tight circular grinds at the base of each cascade wear the rock into the bathtub-shaped depressions that name the falls.

Yes, with traction. Both Arethusa Falls and Diana's Baths stay open in winter, but the Arethusa Falls Trail freezes hard from late December through March and the steeper second half becomes a sustained icy grade that requires microspikes or full traction (trekking poles help, and snowshoes are sometimes needed after fresh snow). The falls itself forms a multi-pitch ice climb rated WI3 to WI4 and is one of the more popular intermediate ice routes in New England. Diana's Baths is workable with microspikes or snowshoes on the flat gravel path; Lucy Brook runs partly under ice and the pools rim-freeze without fully sealing. Both trailhead lots are unplowed; park in the US-302 pullout for Arethusa and use the West Side Road lot at Diana's Baths if it has been cleared.