Rivers are recognized as major unilateral pathways of MP transport between terrestrial and marine ecosystems, yet our understanding of their dispersal over space and through time as they migrate from source to sink is limited. Furthermore, freshwater microplastic studies have largely focused on perennial rivers with little attention to intermittent rivers and ephemeral streams. In this study, surface water and sediment samples were collected monthly from 24 sites along two urban ephemeral rivers (Leon Creek and Salado Creek) in San Antonio between June 2021 and May 2022 to characterize and evaluate the spatiotemporal distribution of microplastics. Microplastics were found in all sites throughout monitoring timeframe (n = 10451). Fibers were the most abundant (~87%) morphology followed by foams (7%). The abundance of microplastics varied from 2 to 320 items/cm3 and 0 to 493 items/kg-1. Potential MPs were marked and analyzed using Fourier Transform Infrared Spectroscopy (FTIR) for confirmation and polymer identification. This study is the first to monitor and report microplastics in ephemeral streams. The global extent of IRES systems is projected to increase with continued climate change; therefore, understanding how hydrodynamic patterns influence microplastic spatial distribution and fluvial transport regimes constitutes valuable information in assessing microplastics pathways and their fate as a part of the global “Plastisphere” geochemical cycle in the Anthropocene.