Grant Lake (#0500)

The short answer is that there's _always_ water underground--the question is how deep! It's a question of the local topography, the permeabiity of the rocks and soil, the amount of precipitation, and so on. I note the aspen seem to be along drainages, which is consistent with the water table being more shallow thru there. But there is also a certain variability in the vegetation cover just from where things happened to take root.
Btw, the ridge in the background is a big lateral moraine, of Tioga age judging by the abundance of white boulders on its surface. (Boulders at the surface break up geologically quickly due to weathering, so the older Tahoe moraines do not have the scattered boulders at their surface.) Anyway, a moraine is a ridge of loose debris pushed out of the way by a glacier, so it's pretty permeable. Hence, the problem will be that there's little barrier to the rain and snowmelt sinking very deep, so there will not usually be a lot of groundwater accessible high on the moraine--one reason it's so barren. Here along the base of the moraine the water table will be much more shallow, simply because there's nowhere left to sink to, and that will help its availability for large plants like trees You can think of the water table as filling the valley here. There will, of course, still be a slow rate of flow of the subsurface water in the valley downstream over time. As a general rule you get surface water where the water table intersects the ground surface.

Hope this thumbnail sketch of Hydrology 101 helps! ;)

It's morning, so this gets me thinking of this in terms of a giant bowl of cereal. Extending this, it seems the bushy growth around the reservoir also indicates the likelihood of groundwater. How would one know whether that was seepage from the reservoir versus down-slope seepage?

The localization right along the shoreline suggests it's related to the reservoir water level. You'd expect vegetation watered by down-slope seepage to be more dispersed vertically, as in fact the aspen are.
One of the things that happens with reservoirs, tho, is that they boost the local water table from water soaking in! This is a loss mechanism that wasn't always accounted for in the old days. I remember reading somewhere that the Navajos along the canyon that Lake Powell filled got a windfall--wells were much more productive in a few years!