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Removal of all organic debris dams from a 175-m stretch of second-order stream at the Hubl;'ard Brook Experimental Forest in New Hampshire led to a dramatic increase in the export of from ecosystem. Output of dissolved organic carbon ( <0.50 J.tm) increased 18%. Fme particulate orgamc carbon (0.50 J.tm-1 mm) export increased 632% and coarse particulate organic matter (>I mm) export increased 138%.Measurement of the standing stock of coarse particulate organic matter on streambeds of the Brook Valley revealed that organic debris dams were very important in accumulating this matenal. In streams,_ debris dams contain nearly 75% of the standing stock of organic matter. The proportion of orgamc matter held by dams drops to 58% in second-order streams and to 20% in third-order streams.Organic therefore: extremely important components of the small stream ecosystem. _They retam orgamc matter w1thm the system, thereby allowing it to be processed into finer size fractions in headwater tributaries rather than transported downstream in a coarse particulate form.