Aquifers: Saviors of Suburban Trout Streams

What are Aquifers?

Aquifers are large pools of groundwater close to the surface. The temperature of this groundwater is roughly equal to the mean annual air temperature and changes little over the course of the year. Most of the wild trout in Eastern and Central Massachusetts rely on these groundwater inputs to survive.

Unfortunately, suburbanization is a major threat to trout streams. Impervious surfaces like lawns and paved areas cause warm rainwater to run directly into streams, rather than soaking into the ground where it can cool down. This can raise water temperatures to levels lethal for trout. Luckily, many of our wild trout streams flow over aquifers, which contribute enough cold groundwater to buffer the effects of runoff. The springs also have the added bonus of keeping temperatures slightly warmer in the winter. I find that the spring-influenced suburban streams fish better in the winter than the more pristine streams in the western part of the state.

Lots of impervious surfaces = no wild trout in my town

How to Find Spring-influenced Streams

As usual, MA OLIVER is a great resource. It provides a map layer called "Aquifers by Yield Green Shades", which is in the "Aquifers" folder, which itself is in the "Physical Resources" folder. This map is pretty self explanatory - green shaded areas are aquifers, with darker shades of green representing those with a higher yield (more water).

You can combine this with the Coldwater Fisheries layer (also in the "Physical Resources" folder) to see which trout streams flow through aquifers. You'll notice that many of the remaining coldwater fisheries in Eastern MA pass through aquifers.

There are also smaller aquifers that aren't listed on the map. Look for areas with large sand and gravel deposits. These are common in large river valleys, especially those, such as the Nashua and Connecticut valleys, that were previously covered by glacial lakes. Groundwater can easily flow between sand and gravel particles, so these areas tend to have springs where streams pass through.

Another tell-tale sign of spring-influence is a small watershed. While freestone streams depend entirely on water that falls as precipitation within the watershed, spring-fed streams are composed of both ground and surface water. This means that a spring fed stream can have the same volume of water in a much smaller watershed. If you see a decent-sized, perennial stream with a small watershed, it's likely it has springs.

A spring given away by melted snow

You can also look for springs as you are fishing. In the winter look for areas where significant amounts of snow have melted. You'll also sometimes see green vegetation in the water near springs long after other plants have died off. Sometimes you can see water seeping into the stream from the bank - this is especially common in areas where a stream cuts into a steep hillside. My theory is that the water table is slightly higher in the hill, and the erosion from the stream allows some of that water to "leak" out into the stream. Individual springs are good spots to fish in the winter, when fish will seek out the warmer water, but leave them alone when water temperatures get dangerously warm - trout will seek these spots as thermal refugia.

Green plants in a spring during winter

The spring from the above picture is in the bottom right corner of this picture - note the melted snow