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Abundance and types of aquntic nlacroinvertebrates present is largely dependent on the nature of the substrate. Complex substrates generally contain the greatest diversity (variety) and abundance of aquatic organisms (Hynes 1970, Allan 1975). Substrates become more complex as the maximum size of the largest stones in a mixture of substrate sizes increases, which lends to greater habitat diversity (Hynes 1970, Reice 1974). Ecosystems exhibiting lack of' species diversity may be unstable (Cairns 1974). The size distribution of the gravels presently being used for riffle restoration in the Trinity River does not form very complex substrates. Habitat diversity is greatly reduced, which results in relatively low abundance and diversity of aquatic organisms. Such a system is unstable, with minor fluctuations in any component of the ecosystem result in^ in radical changes i n the biotic component. Such a system cannot provide an adequate or reliable source of food organisms for maximum fish consumption and survival. It is recomended that future riffle restoration projects or modifications of existing projects incorporate buffer areas of complex substrate composed of gravel sizes up t o those normally found in the Trinity River. Such substrates should favor diversification of the natural fauna of the Trinity River and provide important rearing areas for juvenile salmonids. Studies should be initiated to determine the optimum mixture of gravel sizes necessary to produce the most favorable habitat.