Legolas can easily run across a rope above a stream

‘There is one of my people yonder across the stream,’ he said, ‘though you may not see him.’ He gave a call like the low whistle of a bird, and out of a thicket of young trees an Elf stepped, clad in grey, but with his hood thrown back; his hair glinted like gold in the morning sun. Haldir skilfully cast over the stream a coil of grey rope, and he caught it and bound the end about a tree near the bank.

‘Celebrant is already a strong stream here, as you see,’ said Haldir, ‘and it runs both swift and deep, and is very cold. We do not set foot in it so far north, unless we must. But in these days of watchfulness we do not make bridges. This is how we cross! Follow me!’ He made his end of the rope fast about another tree, and then ran lightly along it, over the river and back again, as if he were on a road.

‘I can walk this path,’ said Legolas; ‘but the others have not this skill. Must they swim?’

‘No!’ said Haldir. ‘We have two more ropes. We will fasten them above the other, one shoulder-high, and another half-high, and holding these the strangers should be able to cross with care.’

When this slender bridge had been made, the Company passed over, some cautiously and slowly, others more easily. Of the hobbits Pippin proved the best for he was sure-footed, and he walked over quickly, holding only with one hand; but he kept his eyes on the bank ahead and did not look down. Sam shuffled along, clutching hard, and looking down into the pale eddying water as if it was a chasm in the mountains.

He breathed with relief when he was safely across. ‘Live and learn! as my gaffer used to say. Though he was thinking of gardening, not of roosting like a bird, nor of trying to walk like a spider. Not even my uncle Andy ever did a trick like that!’

The Fellowship of the Ring, Book Two, Chapter 6: Lothlorien.