The processing of organic matter in stream and downstream transport is a fundamental process in headwater zones. Water withdrawals from a lower order foothill tropical stream (Casanare- Colombia) and effects in discharge magnitude, macroinvertebrate assembly, the quantities of Benthic Particulate Organic Matter Coarse and Fine (CPOM and FPOM) and decomposition rates of Guatteria sp, were studied during four months. Two sampling sections were selected, corresponding to a reference zone (R) without discharge alteration and altered zone (A) downstream of an intake. No evidence of a significant reduction of water were observed (KW= 1.25, p > 0.05, n = 12) and macroinvertebrate composition, diversity and relative abundance of families, genera and trophic groups showed no differences between both sections, while therewere a greater absolute abundance of organisms in the altered zone. The preference of macroinvertebrates by leaf litter substrates and a litter accumulation that was evident in the altered zone explained the greater abundance of specimens in this. Differences in macroinvertebrate composition between rocks and leaf litter substrates suggested its role in structuring the community. Effects on organic matter processing were evident with increased retention of Benthic Particulate Organic Matter (CPOM and FPOM) in the altered zone (KW p menor que 0.05, n = 80) and significant differences in decay rates (ANOVA F = 4.78, p menor que 0.05), being lower in the alteration zone. Decomposition rates were higher in Coarse Mesh bags and lower in Fine Mesh bags on both sections. The accumulation of MOPB, from september to december, was correlated with lower rainfall events during the dry season and the current velocity. Decomposition rates were correlated with dissolved oxygen and conductivity. Macroinvertebrates shredders did not play an important role in the decomposition rates of Guatteria sp and the fast decomposition were probably associated with the current velocity through its effect on drag and physical fragmentation of leaves in Coarse Mesh bags, with oxygen concentrations and temperature, to stimulate microbial conditioning, which has been more important in tropical streams. Decomposition was associated with physicochemical and hydrological parameters rather than biological factors. In fact water withdrawal did not cause significant changes in discharge or affects in macroinvertebrates assembly. However, significant effects were observed in processing organic matter by changes in decay rates and greater accumulation of BPOM avoiding downstream transport.