The physical dynamics in rivers, wetlands, and estuaries drive important geochemical processes, yet our ability to measure these processes is hampered by an inability to resolve complex sources or to sample at temporal or spatial scales appropriate for the system. We have used in situ measurements of optical properties -- spectral properties of absorbance, fluorescence -- at high temporal and spatial resolution to elucidate changes in DOC concentration and composition, to differentiate among sources of DOC, and as proxies for geochemically-related including nitrate, Hg, MeHg, and others. Optical properties are readily measured in situ, yet also contain a large amount of useful information related to sources and geochemical processes. We will present examples of how such measurements can offer powerful new insights into geochemical processes. One example demonstrates how measurements at a high temporal resolution are useful for characterizing processes in a tidal environment. Measurement of biogeochemical variation and constituent mass flux in estuarine environments, where tidal dynamics dominate, is impractical over the diurnal to spring-neap time- scales necessary to accurately quantify flux. We used in-situ optical instrumentation to quantify constituent fluxes as well as examine changes in DOC sources and composition over daily to seasonal scales in a tidal marsh. Another example demonstrates how measurements at a high spatial resolution are useful for quantification of benthic fluxes. We used a combination of grab sampling and in situ profiling of optical and physical properties to estimate benthic flux in a drinking water reservoir. Measured optical properties correlated well with bulk DOC composition, but also provided a basis for deconvolving the measured bulk DOC flux into that from individual sources. Finally, we will present a number of examples showing how variations in optical properties help to identify source, chemical composition, and reactivity, thus augmenting and extending geochemical investigations.Bergamaschi, B.A., B.D. Downing, E. Boss, J.A. Fleck, M. Stephenson, and E. Kalve, 2006. But Are Optical Measurements Good for Anything Practical? 2006 Fall Ocean Sciences Meeting.