The problem of low dissolved oxygen (DO) level has been found to be widespread in ice-covered or polluted rivers. This thesis is targeted at providing some fundamental studies on the remediation measure of injecting air/oxygen via existing effluent diffusers to increase river’s DO level, with two main focuses:​ effluent mixing from multiport diffusers and bubbly jets in crossflows.

A comprehensive literature review was first provided on transport of conservative chemicals with jets and plumes in the environment. Effluent mixing in rivers was studied based on a field dye test, and the mixing was further divided into near-field and far-field mixing. In the near-field (within the vertically fully mixed distance), effluent mixing from a multiport diffuser was studied in four zones:​ free jet zone, jet surface impingement zone, merging zone and river vertical mixing zone. The applicability of prevailing models for multiport diffusers was examined. In the far-field, river transverse mixing dominates further mixing of effluent. A modified streamtube method was proposed to describe transverse mixing. This method can produce a reliable mixing coefficient even with relatively low-quality field data. Effects of river discharge variation and ice cover on dimensional and dimensionless transverse mixing coefficient were investigated in a fixed river reach.

Bubble plumes and bubbly jets produced by injecting air-water mixtures via a nozzle, were studied in crossflows in a large laboratory channel. Their general behaviors, mainly trajectories, were first photographically studied. Air and water discharges at the nozzle and the strength of crossflows were found to have significant impacts on the general behaviors. Proposed models could well simulate the trajectories of both gas-phase plumes and liquid-phase jets in crossflows. Detailed measurements on bubble properties in bubble plumes and bubbly jets in a crossflow were then conducted with a fiber-optic probe. Distributions of gas void fractions, bubble frequency, bubble velocity, bubble diameter and specific interfacial area were obtained at different crosssections. Centerline properties and spreading rates of bubble plumes were investigated. Relation of bubble slip velocity versus bubble diameter for bubbly jets in crossflow was obtained, and compared with those for single isolated bubbles and bubbly jets in stagnant water.