Glymur

From Reykjavik, take Highway 1 north from its junction with Highway 49, and proceed for 24km, then turn right onto Route 47, marked for Hvalfjörður (if you go straight, you'll enter the tunnel under the fjord and bypass the most direct route to the falls). Follow Route 47 for another 35km and turn right just after crossing a bridge where a red and white sign indicates 'Glymur Foss'. Follow this road, keeping left at the junction after 0.75km, to its end in just over 3km at the trailhead. The trail begins by passing through a gate and following a gravel road. Watch for rocks marked with yellow paint to indicate the correct route - there are three or four spots where it gets a little confusing which tread to continue along. After 1.25km of easy walking the trail seems to end just before reaching the river below the gorge. Watch for steps descending toward what appears to be a cliff - the trail actually makes a sharp left bend and passes through a short cave in order to get down the cliff to the river below. At the lower entrance to the cave there are three natural bridges which provide a cathedral-like ceiling. After passing through the cave, the trail follows the river a short distance, then crosses the river on a narrow log with a cable hand rail. After crossing, the trail makes a steep climb out of the canyon with the assistance of rope hand holds on the steep sections (which may not be appropriate for small children) and then climbs more gradually to the first view of the falls after about 1.8km. Better views are had further up the side of the canyon, but the trail gets considerably steeper and less protected after this point. The best views are had at approximately 2.5 and 2.75km from the parking area. Additionally, there is a trail which climbs on the opposite side of the canyon and avoids the cave and river crossing, but the vistas of the falls aren't nearly as good.Glymur is a waterfall of considerable size found along the Botnsá at the head of an impressively deep and narrow canyon at the head of Hvalfjorður, about an hour north of Reykjavik. Flowing from a modest lake across a relatively flat plain, the river encounters the head of the valley - which at first glance from the road does not look like it should harbor a waterfall of this size. Instead of gently cascading down or stair-stepping over several smaller waterfalls, the river has carved what is effectively a slot canyon on a massive scale - nearly a kilometer in length and well over 700 feet deep at its apex.
At the head of the canyon the falls plunge about 650 feet in two primary streams, one accounting for the majority of the flow of the river which skips down a sluice in a narrow horsetail form, and a second which veils out broadly over a ledge with a single step in it. Upon our visit in August of 2014 we attempted to measure the falls but found it was situated too far away to get an accurate reading, however in eyeballing the falls, we find 650 feet to be entirely plausible for its height.
Up until recently Glymur was considered to be the tallest waterfall in Iceland. This changed not because its stature had been mis-represented, but because the retreat of a glacier near Skaftafell - the Morsarjökull - brought to attention a taller waterfall which rightfully had claim to the title. This waterfall, now known as Morsárfoss, has actually existed for decades but has only recently achieved notoriety due to the increased attention being paid to the retreat of Iceland's glaciers.The word Glymur translates roughly to "crashing" or "roaring" in english - which is apt given the echoing roar produced by the falls as it thunders into the deep canyon.