Rivers and Streams

Lotic Systems

The main distinction between a lotic system and a lentic one is the presence in lotic systems of a unidirectional gravity induced current. Another way of looking at a lotic system is to think of it as a series of overlapping lentic systems with very short residence times. In many ways, lotic systems resemble lentic ones, and we will focus here on the differences.

You are no doubt familiar with many of the names given to lotic systems. Generally, in order of increasing size they are: seeps, springs, streams, rivers. There are also creeks, cricks, rivulets, and so on. Be particularly cautious of the habit in the western U.S. to name every drainage ditch a river in honor of the 2 hours every ten years they actually achieve such flows. This practice no doubt grows out of nostalgia for water, and to compensate for the eastern habit of naming large hills mountains.

The current dominates lotic systems. As mentioned earlier, current speeds can get up to about 3 m/s, but most rivers have currents less than 1 m/s; many "fast" streams are actually flowing about 0.3 m/s, and large rivers have deceptively fast currents. Remember also that the average current speed does not take into account the areas of fast flow (near the surface in deep water) and slow flow (along the bottom and edges, behind obstructions) that may exist. Many small streams are actually series of lotic fast water and small pools that are essentially lentic.

Current meters, both mechanical and the time honored timing of an apple (density near that of water, floats but is not affected by wind, bright red, easily visible, cheap, biodegradable, nutritious) over a measured distance, only measure average current speed, and about the only thing you can say about average speed is that it is the speed that most organisms are least likely to encounter as they crouch behind and under rocks, logs, etc. In addition, there is the boundary layer (Fig. 1). As Stephen Vogel puts it, the boundary layer is perceived by most as a fuzzy notion that it's a discrete region rather than as a discrete notion that it's a fuzzy region. The boundary layer is defined arbitrarily by humans; it refers to the fact that a fluid flowing over a surface tends to "stick" to the surface, so that at the surface the fluid speed is essentially zero, and it increases rapidly. Engineers usually define the boundary layer as the area in which current speed is up to 99% of the undisturbed current speed; biologists typically use 90% as the cutoff point (I told you it was arbitrary).

Figure 1. Graph showing height above the substrate where the current speed is 90% of the mean current velocity (1 m/s). Note that the further back one goes from the front surface of an object placed in the current (moving to the right along the horizontal axis), the thicker the layer becomes. Note also that the layer in this example is always quite thin (up to 2 cm here), and that even within the boundary layer the water is anything but stagnant. Current speed will decrease to zero at the water-substrate interface. A higher current speed would result in a boundary layer curve displaced down and to the right.

The boundary layer gets thicker as current speed gets smaller, and gets narrower as you approach the front edge of a surface exposed to flow. The boundary layer is thus wider at the back of a rock in the stream then it is near the front edge, and, if current speed increases (say, after a rain), the boundary layers in both locations will grow narrower. If you doubt the existence of the boundary layer, look at the blades of a fan, and figure out why the dust there doesn't blow off when the fan is turning. The reason, of course, is that the dust is well within the boundary layer and thus not exposed to the air currents. In streams, only microorganisms are small enough to escape the effects of the current by living in the boundary layer; most multicellular organisms are simply too big even if they are highly flattened (Fig. 2) (see McShaffrey and McCafferty, 1987 for more on this). For more on the physics of water, click here.

Figure 2.  Living water penny beetle larvae can control the flow of water - the boundary layer - over their surface.  By doing this they ensure a streamlined flow and reduce drag which would tend to displace them.  Although they control the boundary layer, the larvae are large enough that they extend through it.  The image to the right is a dorsal view of a living water penny beetle larva.

Because of the costs involved in trying to maintain position in a current, most organisms in lotic systems are benthic, and hold onto the bottom in one of the ways mentioned above. In general, benthic organisms in streams have more elaborate holding mechanisms than those in lakes; these holding mechanisms include such things as silk which is used by insects in very fast currents. Organisms in streams also tend to be more streamlined (because of the higher Re - remember) or flattened, however recent studies (McShaffrey and McCafferty 1987, Craig 1990) show other reasons for being streamlined such as the ability to inhabit crevices or to accelerate quickly. Remember too that the direction of the current as it swirls around rocks and other obstructions may not be as unidirectional as the net flow of the stream is. Furthermore, the swirling motion is chaotic, hence unpredictable, for most organisms - they must be prepared for the current to switch suddenly.

A strong current means that plant nutrients and gasses such as O2 will usually be mixed throughout the water column, allowing good gas, nutrient, and waste exchange for those organisms which can keep a grip. Plankton do not do well in lotic systems, however, since they may be swept into areas unfavorable for growth. Attached algae may also suffer since moving water can carry soil particles in suspension, blocking out light and raising the LCP above the bottom. The ability of the water to carry and move particles varies with its speed, and the deposition of those particles, which occurs as the water slows down, affects the distribution of different size particles on the bottom of the stream. Because of the chaotic nature of the currents, soil particles on the bottoms of streams tend to be more patchy and mixed than those in lakes, and will often shift position. The table below lists some of the size characteristics of various soil particles that make up the bottom of streams:

Name

Diameter (mm)

Diameter (phi units)

Boulder

>256

< -8

Large cobble

256 - 128

-8 to -7

Small cobble

128 - 64

-7 to -6

Very large pebble

64 - 32

-6 to -5

Large pebble

32 - 16

-5 to -4

Medium pebble

16 - 8

-4 to -3

Small pebble

8 - 4

-3 to -2

Granule

4 - 2

-2 to -1

Very coarse sand

2 - 1

-1 to 0

Coarse sand

1 - 0.5

0 to 1

Medium sand

0.5 - 0.25

1 to 2

Fine sand

0.25 - 0.125

2 to 3

Very fine sand

0.125 - 0.0625

3 to 4

Coarse silt

0.0625 - 0.03125

4 to 5

Medium silt

0.03125 - 0.015625

5 to 6

Fine silt

0.015625 - 0.0078125

6 to 7

Very fine silt

0.0078125 - 0.0039063

7 to 8

Coarse clay

0.0039063 - 0.0019531

8 to 9

Medium clay

0.0019531 - 0.0009766

9 to 10

Fine clay

0.0009766 - 0.0004883

10 to 11

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It is possible to divide the stream into different habitats; these habitats differ mainly in current speed and the resulting nature of the substrate (Fig. 3). Areas where the current is capable of lifting particles from the bottom are known as erosional habitats; areas where particles are coming out of suspension are known as depositional habitats. The distinction is a bit fuzzy, since it really depends on what size of particle you are talking about; usually we refer to the erosional or depositional areas in terms of silt or fine sand. Among erosional habitats, we can distinguish riffles, where at least some rocks break the surface and there is active mixing of air and water, and runs, where the water moves faster but is deeper than the rocks, and which has cobble or larger particles for substrate. Depositional areas are typically called pools, with bottoms of sand or silt.

Figure 3. Habitats in a river (above). Riffles occur in shallow areas where rocks penetrate the surface of the water. Runs are deeper areas of swift current, usually upstream or downstream of a riffle. Pools are areas of still water. In rivers, erosion usually takes place on the outer edge of a bend, deposition on the inner edge. Water is usually deeper on the outer edges of a bend. The image below, of Sugar Creek in Indiana, shows a series of riffles and runs. The cobble substrate apparent in the areas with smooth surface waters suggest relatively high current velocity, thus these areas are better described as runs than as pools. Note the importance of vegetation in stabilizing a bank.

Areas of a stream may switch between being depositional and erosional as more or less water enters the stream. It is important to remember that streams are dynamic, constantly changing. Periods of high and low water may be seasonal or daily depending on such factors as climate, nature of the watershed, and the need for electricity in Las Vegas. For instance, streams in wooded areas are more resistant to flooding than streams in urban areas because the trees, with their leaves and roots, slow the movement of water, keeping it from running off and releasing it slowly; water runs right off urban streets.

Streams differ from lakes in terms of the effects of the watershed or catchment on the biological processes. While lakes are largely influenced by their catchment, they are influenced to a lesser degree than are streams, since the lakes can build up reservoirs of important nutrients and other materials. Streams, on the other hand, may rapidly lose any nutrient that escapes into the water. The fact that many nutrients are carefully conserved as they flow through steam ecosystems has opened a "hot" area of stream research into what is known as nutrient spiraling. Streams are often shaded by trees on the banks, and thus little photosynthesis can occur. These streams are highly dependent on outside energy sources (as distinct from the sun); this outside material is known as allochthonous as opposed to autochthonous materials derived within the stream itself. Streams are often much more dependent on allochthonous sources (such as plant leaves each fall) than are lakes. Note too that the presence or absence of trees will affect the temperature characteristics of the stream; clearcutting of forests not only increases the amount of soil that rushes into a stream, it also warms the stream up, reduces the amount of O2 present, increases BOD, and so on.

As we have already mentioned, streams vary in size. Generally speaking, as a stream increases in size, it will carry more water at a higher speed, be more turbid, be deeper, and be more saline. Riffles and pools will be replaced by long runs. The progression from small headwater streams or springs to great rivers with deltas and distributaries carrying the water into the ocean, is orderly. So orderly, in fact, that a deceptively simple scheme for classifying rivers has come about (Fig. 4). River order is calculated as follows: The first discernible stream that forms (from a spring in most parts of the world) is 1st order. When two 1st order stream merge they become 2nd order; when two 2nd order streams merge a 3rd order stream is formed. A first order stream entering a 3rd order stream has no effect. The problem comes in when trying to pin down those first streams, which may dry up from time to time. Since river orders are usually calculated on maps, the resolution of the maps used is also critical. Generally, first order streams are the ones that show up on United States Geographical Survey 7.5o topographical maps or "quads". Drainage patterns of rivers may often be distinctive due to the underlying geology.

Figure 3. Determination of river order. The smallest streams are all 1st order, when two 1st order streams join they form a 2nd order stream; when two 2nd order streams join they form a 3rd order stream, and so on. Note that addition of a lower order stream does not change the order of the stream; stream order increases only when two streams of equal order join.

As you may have guessed by now, the distinction between lotic and lentic systems is very fine. One of the striking features of the natural world is its firm rebuffs to those who want to divide it up and classify it in neat packages. Most systems are not composed of discrete units but instead are continua, and the lotic - lentic continuum is one of those. As we have seen, even in an otherwise lotic system there are small pockets of calm water which are essentially lentic. On the other end of the scale, very large lentic systems can resemble lotic ones. For instance, Lake Erie is lentic, yet the western basin of the lake, which is very shallow, resembles a large river in some aspects of its biota. Many of the species present are more at home in rivers than in a lake, with the wave-swept shores being most river-like in respect to their flora and fauna.

Stream Tour

 

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