Provided herein are devices and methods for generating microfluidic gradients, including an array of unique microfluidic gradients within an array of microchannels. Fluids within conduits are mixed in an intersection region to generate a mixed flow stream in a source reservoir channel that provides a gradient that varies with axial distance from the intersection region. Microchannels having an inlet connected to the source reservoir channel are configured to provide a microfluidic gradient in the microchannel. An outlet end of the microchannel is connected to a sink reservoir channel. By varying the ratio of fluid flow rates from the fluid conduits, the microchannel gradients are tuned. In this manner, a large number of unique gradients or array of microfluidic gradients is provided, wherein the gradient can be any number of physical or chemical parameters, including concentrations and physical fluid properties.