A method of fracturing a formation with a fracturing fluid in a wellbore extending into the formation includes pumping a spacer fluid through a spacer fluid channel into the wellbore and pumping a proppant slurry through a proppant slurry channel into the wellbore separately from the spacer fluid. The method further includes combining the spacer fluid and the proppant slurry within the wellbore to create the fracturing fluid with portions comprising varied proppant concentrations and pumping the fracturing fluid with the varied proppant concentrations into the formation.