As eeeek wrote it be cold. I've fully dunked myself in Lower Cathedral Lake at the eastern granite "boardwalk" and can readily attest to that fact.
In fact almost all the thousands of lakes and creeks in the High Sierra stay too cold for most people to stay in the water very long throughout the summer. When I say High Sierra I mean lakes above say 9k like the Cathedral's. I must admit to wishing this wasn't so because the water is often so beautifully clear and enviting. Personally many of our streams with rocky potholes and rocks to dive into pools from interest me most. Of course there are a number of fine natural lakes to swim in at lower elevations, especially in the Central and Northern Sierra about the 5k to 8k elevations in lower glaciated bedrock geology.
Lakes of course warm up slightly during days and cool down during nights. Over a period of weeks during the summer, the water temperatures peak sometime in August before the waning sunlight hours, and lower sun altitudes change the balance. There are lots of environmental location factors that effect lake temperatures, especially cold inflow temps and volumes of water from streams, but also wind which stirs up deeper waters, and shadowing by adjacent landscapes. Given the same environments, shallow lakes will tend to warm up faster over time than deep lakes. Even in the most sun warmed water, there is often a significant thermacline with the surface just cool while a mere few feet below is much colder. And there are thousands of shallow ponds in the high country and certainly some that warm up enough by August to make swimming practical. The ideal situation would be a shallow pond amid dark metamorphic rock, in a wind protected, sunny exposure pocket canyon, with a small clear stream inflow. There are of course other shallow stagnant ponds in the High Sierra, often in timbered areas that start clear after snows melt, then over weeks become disgusting seething increasingly dark brown stews of all sorts of biota. Warm indeed but to be avoided.
I've backpacked in the Sierra Nevada for over three decades now averaging about five trips each year. On almost all those trips, I will usually jump into some lake or stream at least once a day for a very brief few seconds before the cold has a chance to chill my core, as I greatly enjoy the refreshing feeling of being clean. Especially later in the evening when retiring into my sleeping bag. Such of course runs counter to the majority of backcountry users that absolutely refuse to suffer in chilly waters.
...David
http://www.davidsenesac.com
http://www.davidsenesac.com
