The objective of this study was to elucidate the nature and extent of drift by the aquatic insect populations of the Brazos River, Texas.

Users of the Instream Flow Incremental Methodology CTFIM) commonly assume that there is a positive linear relationship between available habitat (WUA) and stream fish biomass. However, several studies have shown a lack cf correlation between WUA and biomass. Such poor relationships may occur if other factors limit fish abundance. Food availability is a potentially important factor limiting abundance of stream fishes and can affect blomass by influencing fish condition (individual weight). I collected rainbow trout (Oncorhynchus mykiss) exposed to different levels of invertebrate-drift density to determine if condition was greater where food was more abundant. I also examined whether size-selective predation and gut fullness varied with drift density. Size-selective predation was not related to drift density. However, condition and gut fullness were both significantly correlated with drift density. Furthermore, changes in drift density appeared to have a stronger influence on small-trout condition than large-trout condition. The results of this study and others support the contention that the reliability of the IFIM might be improved if food availability were incorporated into models.