The temporal variations in the Earth's gravity field caused by fluctuations in terrestrial water mass can be inferred from changes in GRACE monthly gravity field solutions. Such methods have limited spatial resolution due to a necessary and possibly arbitrary truncation and smoothing of the coefficients. Limiting the temporal resolution to one month was necessary to solve the gravity field by a global representation. Our alternative method uses GRACE satellite-to-satellite tracking and accelerometer data to obtain the along-track geopotential differences and directly estimate the temporal gravity variations regionally. This method was tested on the estimation of hydrological mass anomaly over the Amazon and Orinoco river basins. Compared to conventional spherical harmonic methods, the spatial extent of the estimated GRACE water height anomaly achieved finer resolution and is shown to follow more closely the boundaries of the river basins and significant systematic variation could be discerned at 15-day temporal resolutions.