We started walking across 430-foot-high O'Shaughnessy Dam just after 8 a.m. Saturday, and there was a yellow metal sign with black letters that said, “WARNING Dangerous High Water at Wapama Falls Footbridges Cross at Your Own Risk.”

This was a week ago, and the weather was clear and sunny the entire time we were out there at Hetch Hetchy. There was water on stretches of the dirt-and-rock floor of the dark, hundred-yard-long tunnel on the far side of the dam.

We came out of the tunnel and one of the first things we noticed were stands of brilliant purple lupine, fresh and succulent, white and violet against the gray-green foliage from which they grew. They clung to edges of the trail we followed around the top of the bath-tub ring that shows on the cliffs enclosing Hetch Hetchy Reservoir.

The surface of the man-made water storage container, first dammed in 1923, was about 50 feet below its maximum elevation of 3,796 feet, according to USGS data for May 11 and USGS mapmakers. The giant reservoir, which stretches eight miles back into Yosemite National Park wilderness, was holding 251,000 acre-feet of water, 69.9 percent of it capacity.

The surface of the water was deep, steely dark blue and gray, rippled in places by winds coming out of the Grand Canyon of the Tuolumne River and off the Central Sierra crest that bounds the highest edges of the 459-square-mile watershed impounded by O'Shaughnessy Dam.

Walking farther along the trail toward Wapama Falls, we could no longer see the 1,080-foot-tall cataract, now hidden by cliffs as we approached it. We could see multiple other smaller streams and creeks and falls, though, and we had to cross each one on the way.

The drip-drop, splish-splash flows of these snow-fed creeks fed islands of marshy grasses isolated on level sections of smooth, glacier-polished granite. The ice-cold waters congregated in places and close by there were crowds of white flowers known as meadowfoam. Some jagged-edged rocks and boulders sported moist, rusty orange-red hairy mosses.

The most water-charged waterfall we could see was Tueeulala Falls, and below we crossed and walked past numerous shallow flows and came across small ponds teeming with tiny insects, spiders and swimming creatures smaller than baby toenails. We came across sunbathing lizards, too.

Wapama Falls was howling and rumbling and roaring louder than a freight train. Spray was coming off it and hitting us before we could see it. Ripples and whitewater from its discharge were visible on the reservoir’s nearly still surface before we could see Wapama Falls.

We pulled on rainjackets and hoods and walked the stony path toward the first footbridge. A section of the wood railing on the upstream side was missing, and metal cables flapped loosely in its place. Some of the waterfall splashed across the bridge and soaked our feet and ankles and shins.

The howl and roar and freezing cold waters of Wapama Falls — and their proximity thanks to the footbridges — were without question the most impressive force of nature we encountered that day.

On the far side of the falls we came to a few patches of pink-red vegetation that turned out to be miner’s lettuce, according to Forest Service biologists. Historians say Gold Rush miners in the Central Sierra used to eat the fleshy, herbaceous plant to fight scurvy, a disease that killed many thousands of malnourished miners and sailors who were cut off from access to Vitamin C.

From the mid-1500s to the mid-1800s, ship captains and shipping investors could assume 50 percent of their sailors would die from scurvy, according to the Science History Institute of Philadelphia. Scurvy killed more than 2 million sailors between Columbus’s first trans-Atlantic voyage and the rise of steam locomotive engines in the mid-19th century.

We could also hear and see fast-moving swifts or wrens, small cavity-dwelling birds who live and hunt by the cliffs east of Wapama Falls. The birds chirped and dive-bombed each other. We tried to dry out on sun-soaked, jagged-edged boulders, surrounded by rock butterweed — yellow blooms, some with red, sun-scorched centers, that were populated with tiny bugs, worms and flimsy, fresh-strung spiderwebs.

We walked back across the Wapama Falls footbridges to enjoy more sound and fury. We also got showed up by three brave young women who happily crossed the bridges wearing tank tops and yoga pants, not hoods and raingear like us. It was still before noon, but the day was warming up at Hetch Hetchy.

We took the walk back slow. We came across more meadowfoam, and pockets of red paintbrush or prairie fire. At one of the creeks we found scores of young butterflies congregating on moist ground next to one of the shallow flows. They were either drinking or sunbathing or both. Some of the butterflies had light blue wings. Others had wings colored black and orange, with light beige spots, and they looked like young Western monarchs.

As we approached the east end of the tunnel, we noticed a sign we hadn’t seen before. It said, “My Ancestors Live Here,” and it continued, “For generations, Central Me-Wuk, Southern Sierra Miwuk and Mono Lake Paiute people made mountain places like this their homes, using the higher mountains for trade routes and summer encampments. We have lived on the land as hunters and gatherers, and traditional skills and beliefs were passed on. We still live here and continue to teach our children the ways of our ancestors.”

Reporter’s note: Before I began working for newspapers in the early 1990s, I spent seven years with VisionQuest and Outward Bound as a paid, certified wilderness instructor and emergency medical technician accountable for groups of felony offender teens, court-ordered children and adult Cuban refugees. I am in my late 50s and anyone who walks OK on their own can keep up with me anywhere. This account is not to be taken as an endorsement or recommendation of any kind.

Contact Guy McCarthy at gmccarthy@uniondemocrat.com or 588-4585. Follow him on Twitter at @GuyMcCarthy.