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80 rubble, gravel, and sand. Compared to headwater reaches, the ratio of shore line length to streambed surface area is reduced because of increased channel width in a downstream direction. SInce midreaches are generally more than 10 meters wide the riparian canopy does not close over them, permitting a significant increase in solar input. Ground water input has a lessened effect on water temperature becausE' increased water volume and increased solar radiation result in greater daily fluctuation of water temperature than in headwater streams. Geographical location of these streams and the effects of altitude, latitude, and longitude have a pronounced impact on where transition zones may occur between warm and coldwater types of habitat. The collection of water from the coalescing network of feeder tributaries results in an increased concentration of nutrients (Figure 3). The net effect of channel widening is a smaller riparian influence and a reduction in allochthonous organic input. Water depth tends to remain shallow, especially in riffle areas where the effect of sunlight (radiation) is maximized. Therefore, the major trophic resources are a result of fine particulate organic matter input from upstream and instrea::J organic matter production by primary producers. In con~rast to heavily shaded headwater streams with a P/R ratio of less than one, the ratio for medium-size streams is greater than one. Decreased allochthonous input and a significant increase in autochthonous primary production resul t in an autotrophic community structure. Primary production in excess of eommunity consumption is available on an annual basis for export downstream or storage in sediments. Autochthonous primary production: by (rooted) resident plant life consistently generates fine particulate organie matter (FPOM). At death, large amounts of coarse particulate organic matter (CPOM) exist in the form of fragile aquatic plant tissue that readily decomposes into FPOM by mechanical abrasion 'lIld microbiological processin", Large Streams Large streams are described by a number of easily observable physical characteristics that more or less begin in the vicinity of seventh order reaches and extend to the mouth or estuary. Channels progressively increase in size becoming wider and deeper (Figure 2). Gradient or slope is generally less than in midreaches. Volume of water may be significantly increased from inflowing tributaries. Channel substrate is generally dominated by fine sand, silt, and clay, some of which is highly tr<lRsient and constantly being carried in a do...n stream direction. Water temperature in large streams may vary between cold and warm depending upon water sources, altitude, latitude, longitude, and volume. Short-term water temperature fluctuation is generally moderated by the large water volumes. Stream width precludes any significant shading from adjacent riparian vegetation, allochthonous organic inputs are low, and the riparian influence is small as a result of a low rati,) between shore line length and streambed surface area (Figure 3). ...lthough not studied, side tributaries may be a source of some CPOM, but trophic inputs are dominated by FPOM imported from upstream. Primary production 15 generallt low and limited by