National Geographic published this photo of trees covered in spider silk in Pakistan. This area was ravaged by major floods last year, and as waters receded, these trees became one giant web.
According to the article, floodwaters "drove millions of spiders into the trees," but this is highly unlikely. For starters, most spiders are not at all social. Another spider is just something to attack and eat. A tree full of stranded spiders would be a scene of carnage. Secondly, spiders build many different types of webs, but each species builds in a distinct style. These trees are almost surely colonized by a single species, not a collection of flood refugees.
There are a few species of social spiders, even one that occurs locally. I've seen their webs along Third Creek. They build a messy glob of silk most spiders would be ashamed of on branch tips over water. They look like the work of caterpillars, and these photos look like cherry trees infested with tent caterpillars.
The article includes no photos of the spiders, but I'm sure NatGeo is reliable on that account. My guess is these webs were built by social, riparian spiders. Rather than being driven to the trees by the flood, colonies probably experienced a population boom feasting on flies that feasted on the detritus of the flood.