At the heart of Severson Dells Nature Preserve is the valley of Hall Creek, with its weathered bedrock cliffs standing as mute testimony to the power of moving water. Thousands of years ago, glacial meltwaters carved the valley, exposing the ancient dolomite (a kind of limestone); today Hall Creek (or Mosquito Creek as it is called out on some maps) meanders through the valley, flowing year-round, draining farm fields that occupy areas north of Montague Road and carrying the water to the Rock River.

 Hall Creek, flanked by dolomite limestone cliffs

Hall Creek, flanked by dolomite limestone cliffs

Within the field of physical geography, more specifically fluvial geomorphology, Hall Creek would be categorized as a ­first-order stream or a headwaters stream. What does that mean? Well, human beings are always sorting, classifying, or categorizing things. Taxonomy generally refers to the classification of organisms, but the same principles apply to other things, including streams. Stream order is a way of sorting waterways—from small headwater streams to mighty rivers.

To explore this idea, we first need to settle upon a definition of stream. We’ll use the term in a generic sense to refer to any body of water that flows, generally confined within more-or-less well defined banks.

Early classification systems tended to be subjective and therefore inconsistent. Classifications based upon morphology (such attributes as gradient, sinuosity, width-depth ratios, etc.) had limited application because stream characteristics may change back and forth across space. (Sinuosity, in case you were wondering, is defined as the ratio of channel length to valley length.) In the words of hydrologist David Rosgen, “One consistent axiom associated with rivers is that what initially appears complex is even more so under further investigation.”

So, based on work done in the middle 20th century by researchers named Horton and Strahler, stream order can be quantified by what is known as a Strahler Number (or Horton-Strahler Number). It works like this. A perennial headwater stream is a first-order stream. Below the confluence of two first-order streams is a second-order stream. Below the confluence of two second-order streams is a third-order stream. This continues until the 12th order.

(Note that adding a first-order stream as a tributary to a second-order stream does not elevate the second-order stream—it continues as a second-order stream and the first-order stream ceases to have its separate existence. It is only when encountering another second-order stream that the order is elevated to third. And so forth.)

 an illustration of the Strahler numbering system

an illustration of the Strahler numbering system

Generally, first-, second-, and third-order streams can be considered small or headwater streams; those ranked fourth through sixth are considered medium-sized streams; those ranked seventh through 12th are considered to be rivers. For example, the Ohio River is an eighth-order stream; the Mississippi is a tenth-order stream; the Amazon is a 12th-order stream. Some 80 percent of the world’s waterways are small (first- to third-order) streams.

To further confuse things, there is an alternative method, the so-called Shreve method, which is additive: the confluence of a first-order and second-order stream yields a third-order stream; the confluence of a third-order and second-order stream results in a fifth-order (or fifth-magnitude) stream. Because the Shreve method counts all upstream tributaries they may be referred to as magnitudes rather than orders.

 An illustration of the Shreve numbering system

An illustration of the Shreve numbering system

So what about all the other names we have for streams (like creek or brook)—do they have specific meaning? One way of thinking about it is expressed in an old adage:  “You can step over a brook, jump over a creek, wade across a stream, swim across a river.” Not very scientific, though.

As it turns out, some of the place-names (toponyms) widely used for these flowing bodies of water have certain regional or cultural affiliations. For example, words like rio and arroyo are of Spanish origin and are commonly used in the Southwest. Here in the upper Midwest we have several other words for streams, mostly borrowed from North, South, or Midland sources.

Brook, for example, is widely used in New England and elsewhere in the North. Branch, on the other hand, is used mostly in the South. Run is a Midland term. Such words may also be used to describe the physical milieu of the stream. In that context, a run implies a swift flow, perhaps across relatively steep terrain, with energetic action, while a branch or fork may suggest divisions of a stream, often in flat terrain.

Creek originally referred to coastal inlets and tidal estuaries, but now is ubiquitous and, much like stream, might be used in the generic sense. Where pronounced, “crick” it likely is of southern origin. Watercourse actually refers not to the flow of water but to the channel it occupies.

So, looking at our place-names, it would appear that whoever had naming rights used terms familiar to them—likely terms that reflected their personal geographic and linguistic histories.

No matter what we name our streams, or how we rank and categorize them, the fact of the matter remains that surface waters are vital components of our hydrological systems and water quality is essential to life. We depend on our waterways to alleviate flooding, recharge groundwater, maintain wildlife, and generally enrich our lives. In fact, streams and rivers do help to sustain us.

P.S., for those readers who would really like to geek out on a map and commentary regarding toponyms applied to streams in the continental United States, I invite you to spend some time here: