A sediment budget of Pegasus Bay is presented in order to build up a comprehensive account of sediment movements and potential shoreline behaviour of the region. The bay has been extensively researched on a local scale but very few works examine the bay as a entire unit. This thesis gives an overview of the bay as a whole as well as examining site specific areas which leads to the derivation of a sediment budget for Pegasus Bay. The principal objective is to establish and quantify the sources, sinks and transfers of sediment within the Pegasus Bay coastal system. This is achieved by establishing annual river yields, estuary and lagoon sedimentation rates; profile volume analysis; longshore transport calculations based on wave observations; and examining sedimentation on the Canterbury Continental Shelf. It is found that the Avon-Heathcote Estuary and Brooklands Lagoon are sinks of sediment, storing 3,000m³.yr⁻¹ and 1,000m³.yr⁻¹ respectively. The annual river sediment yields are found to be 3,005,000m³.yr⁻¹ with the Waimakariri River contributing 78%. A predominant southerly component of longshore transport is discovered in the south (281,000m³.yr⁻¹) and mid Pegasus Bay (l,572,000m³.yr⁻ ¹) while north Pegasus Bay exhibits net northerly transport (203,000m³.yr⁻¹). The onshore/offshore exchange examinations reveal that there is net onshore transport for the bay of 1,190,000m³.yr⁻¹. The gross sediment budget for the region sums to 20,571,000 m³.yr⁻¹. The net sediment budget for Pegasus Bay at 5,843,000m³.yr⁻¹ represents 30% of the gross budget of which rivers are the most significant source contributing 55% to the net budget. The net budget exhibits a surplus of sediment for the each sector of the bay. This is not reflected in five sectors which are in either an erosionary or equilibrium state. Despite this anomaly a positive correlation between the state of the beach and the amount of surplus for each sector is identified. The relationship is represented by a critical line which marks the point at which a beach may change its state.