Technical Abstract: Understanding flow pathways and mechanisms that generate streamflow is important to understanding agrochemical contamination in surface waters in agricultural watersheds. Two environmental tracers, oxygen-18 and electrical conductivity (EC), were monitored in tile drainage (draining 12 ha) and stream water (draining 660 – 5700 ha) from 2000 to 2008 in the semi-arid agricultural Missouri Flat Creek watershed, near Pullman Washington, USA. Tile drainage and streamflow generated in the watershed were found to have baseline oxygen-18 of –14.7‰ year round (which only changed during episodic events). Winter precipitation accounted for 67% of total annual precipitation and was found to be the primary contribution to streamflow and tile drainage, while summer precipitation did not contribute appreciably. “Old” and “new” water partitioning in streamflow was not identifiable using oxygen-18, but seasonal shifts of nitrate-corrected EC suggest that shallow soil pathways contributed significantly to streamflow generation during winter (mean EC 200 'S/cm), while deep soil pathways primarily generated summer streamflow (mean EC 250 'S/cm). Using summer isotopic and EC excursions of large-catchment stream waters from tile drainage, summer in-stream evaporation fractions were estimated to be from 20% to 40%, with the highest evaporation occurring from August to October. Seasonal watershed dynamics and streamflow generation in the MFC watershed may represent these processes at larger watershed scales in the Palouse River basin. A steady 0.9‰ enrichment in soil water oxygen-18 from 2000 to 2008 may be evidence of the hydrologic influence of the Pacific Decadal Oscillation in the Inland Northwest.