Drawing on expeditionary diaries, official correspondence, Indigenous-authored petitions, and incident reports, this article argues that between 1771 and 1783, the Quechán and “Maricopa” alliance networks controlling the Lower Colorado and Gila Rivers compelled Spanish officials to seek diplomatic entry points into a riverine “native ground.” Contemporary studies of late Bourbon Indian policy foreground colonial officials’ negotiations and treaty making with the Native populations that dominated northern New Spain. However, this scholarship has never systematically engaged with Spanish mediation between Indigenous peoples. As this article demonstrates, missionaries and soldiers brokered Indigenous peace agreements to protect overland communication between Sonora and Alta California and stake out a role for the empire in the river region. In turn, Native peoples recruited Spanish mediation to ameliorate the negative effects of the colonial captive trade. Across the eighteenth century, the river peoples had accessed Spanish trade networks by raiding their rivals to furnish colonists with captive labor. Captive raiding exacerbated intra-Native competition and profoundly disrupted daily life. The river peoples consequently used Spanish peace brokering to renegotiate regional hierarchies and stabilize their communities while retaining access to colonial markets. Centering this history of violence and mediation reorients the literature in two important ways. First, it highlights Native geopolitical dominance on the Colorado and Gila Rivers, which endured until the mid-nineteenth century. Second, it reveals that Indigenous mediation constituted one dimension of late Bourbon Indian policy on “native ground.” Future studies may uncover similar practices across the Spanish Empire.