Aquifers can be confusing. Often times, people may find that their well water is going dry or that there isn't much water pressure, though the pump isn't any different than it always has been. If they ask what the problem is, they might be told that it is due to the state of the aquifer. Unless a person knows what the aquifer is and how such things as the snowpack can replenish it, saying that the problem is because of the amount of water in the aquifer probably isn't going to make a lot of sense.
Simply put, an aquifer is a layer of water held up by impermeable rock. It can have rock above it as well, though this isn't necessarily needed. In fact, many lakes are formed by a surface aquifer. Wells often function because holes have been drilled down to the aquifer so water can be pumped out. The wells can cause some problems though. An aquifer usually contains a huge amount of water. Wells can draw the water level down to the point that wells can go dry if too many people use them and if the water isn't replenished.
Aquifers can be represented by surface water; however, a great deal of the water is held between layers of rock where it isn't in plain sight. The structure is usually straightforward though. The bottom layer of rock is normally impermeable, meaning that water doesn't soak through it easily. Seasoned concrete is a form of man-made impermeable rock, which is why it is used in dam construction.
Above this layer is the layer of water, and above that is one or more layers of semi-permeable rock. This is usually softer rock that will allow water to seep through from above. Examples of this type of rock include sandstone, limestone, shale and even slate, because of the faceted plates it is normally found in.
The two layers of rock may be close together, yet the aquifer can hold an enormous amount of water because of the area it can cover. It isn't unusual for an aquifer to contain millions of cubic feet of water. It can take a great deal to draw the water level down. Still, when it happens, it will take a great deal to refill the underground reservoir, too.
The problem is that semi-permeable rock doesn't really let the water run right through. Rain, landing on even soft sandstone, moistens the stone, however most of the water runs off before it can sink through to the aquifer. This is where snowpack comes in and why it is important.
A snowpack is much as it sounds. It is an accumulation of snow that has been packed down, such as by its own weight. Very generally, it is a term sometimes used to describe the on-the-ground total snow. This isn't quite accurate because there can be an accumulation of several feet of powder snow without the snow yet having the chance to pack down. It is still useful to think about a snow pack as being the total amount of snow that is on the ground.
Different kinds of snow have different water densities. For instance, powder snow has a low amount of water in it while wet snow can have a great deal. Also not very accurate, because of this, it is useful to figure that a foot of snow is about the equivalent of an inch of rain. There is a huge difference though.
Snowpack melts gradually. The more packed it is, the longer it takes to melt, as a rule and at the same ground temperature. This means that unlike rainwater that mostly runs off of semi-permeable rock layers, the water produced by snowpack is normally sustained long enough to seep through the rock, to the aquifer.
Many aquifers rely almost entirely on the melting snowpack for the means of replenishing water levels. In those areas, if the snowpack is unusually low, this fact can be one of the earliest signs of an impending drought. A great snow pack though, can greatly help with filling the aquifer.
When it comes to replenishing an aquifer, a snow pack is a major way it can be done, and it tends to be more efficient than rain. This can be best seen in places like the Denver Basin aquifer.