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The following is an excerpt from Poole and Berman (1999): Pathways of Human Influence on Water Temperature Dynamics in Stream Channels.

" Stream characteristics that influence the rate of  heat exchange with the atmosphere can be said to insulate the stream. These characteristics include the height, density, and proximity to the channel of riparian vegetation and the width of the stream channel. Riparian vegetation shades the stream, blocking solar radiation from reaching the channel and reducing the heat load to the stream (Hostetler 1991, Naiman and others 1992, Davies and Nelson 1994, Li and others 1994). Vegetation also reduces wind speed across the stream channel thereby trapping air against the water surface. This action reduces conductive heat exchange with the atmosphere by decreasing convection and advection of heat energy to the water surface (Naiman and others 1992). Width influences channel surface area across which heat is exchanged; a greater surface area allows for more rapid conductive heat transfer. Under the same climatic conditions, narrower, deeper channels will not exchange heat with the atmosphere as rapidly as shallow, wide channels. Similarly, riparian vegetation of a given height will shade a larger percentage of a narrow channel than a wide channel."