What are its boundaries? Where does it begin? Where does it end? As it is described in Maine laws, a "stream" is a complex concept, modified by many qualifying terms that help in regulating abuses and uses of it, but an actual stream is even more complicated than its legal definition suggests.
Watershed to stream bed
Is the boundary of the stream the edge of the water, where you start to get your feet wet in soggy soil, the area that floods in heavy rains, or all the land that contributes surface runoff or groundwater to that stream?
It is all of the above.
The land area from which water drains into a body of water, either above or below ground, is called its watershed. The watershed may cover many square miles, extending far beyond the sounds of a brook or the smell of damp earth at the water's edge. Anything that affects the watershed will eventually impact the stream. A stream that winds through a wooded watershed, for example, will be a much different stream than one that flows through parking lots or golf courses.
The borders of a stream are much broader than they appear to be at first sight.
Clouds, rain and runoff
Have you ever been in the woods during a storm and watched where the rain goes? Some falls directly onto a stream, but most of it falls on the land. There it collects in depressions and sinks slowly into the soil or evaporates into the air. In a hard rain the water overflows into rivulets that race downhill to streams or pond. The water that percolates into the soil then becomes groundwater. It trickles through pore spaces in sand or gravel or between fractures in rocks to discharge into a spring or a stream.
Plant roots absorb some of the groundwater, pull it up their stems and trunks to their leaves, and release it into the atmosphere. There it joins other water vapor evaporated from streams, lakes, or the ocean. Water droplets condense around microscopic particles of dust and salt to form clouds. When the clouds become saturated, the water falls back to earth as rain or snow and rejoins the stream's journey to the sea. In Maine, over 33,000 mapped miles of flowing water are part of this process that has been recycling water for billions of years. This process is called thehydrologic cycle.
From its source to the sea
Streams are like the capillaries and blood vessels that connect to the major arteries, the rivers. But unlike our body's circulation system, the smaller channels deliver most of the water and food to the bigger ones. Without feeder streams, our rivers would not exist.
You could say a stream begins at its headwaters, often in the mountains, fed by an underground spring or the runoff from rain and snow melt. Rivulets of water flow downhill, merging together to become a stream which continues, mixing with other tributaries, until they all become a river flowing to the sea. Here in Maine the mouth of a river usually opens into the ocean in a broad bay where fresh water and salt water mix, called an estuary. The length of a stream may be only a few feet from where it emerges until it joins another stream, or it may traverse hundreds of miles, from the mountains to the sea. Some streams flow year-round, others only after a storm or when snow melts in the spring.
What could be more dynamic than a stream? It is constantly changing its flow, its depth, even its bed, as anyone knows who has observed a stream in different seasons or at different places along its course. It scours, shifts channels, meanders, floods, erodes, carries and deposits silt. Squeeze a stream in one place, and like a water balloon, it bulges in another. Where it is restricted, the stream speeds up to compensate, eroding downstream banks or spreading out to flood adjacent property.
Many factors shape the character of a stream as it progresses from its headwaters to its mouth: the slope and current, the amount of water being transported, its temperature and water chemistry. These, in turn, influence the vegetation, the animals, the bottom sediments, and the shape of the channel at any point along the stream's journey.
So just what is a stream?
Its boundaries are as wide as its watershed, as long as the entire river system from source to sea, and as fluid as the water cycle itself.