<P> Hapua or river - mouth lagoons form in micro-tidal environments . A micro-tidal environment is where the tidal range (distance between low tide and high tide) is less than two metres . Tidal currents in a micro-tidal zone are less than those found on meso - tidal (two--four metres) and macro-tidal (greater than four metres) coastlines . Hapua form in this type of tidal environment as the tidal currents are unable to compete with the powerful freshwater flows of the rivers therefore there is no negligible tidal penetration to the lagoon . A fourth element of the environment in which hapua form is the strong longshore drift component . Longshore or littoral drift is the transportation of sediments along the coast at an angle to the shoreline . In the Canterbury Bight coastal area; the dominant swell direction is northwards from the Southern Ocean . Therefore, the principal movement of sediment via longshore drift is north towards Banks Peninsula . Hapua are located in areas dominated by longshore drift; because it aids the formation of the barrier behind which the hapua is sited . </P> <P> A hapua also requires sediment to form the lagoon barrier . Sediment which nourishes the east coast of New Zealand can be sourced from three different areas . Material from the highly erodible Southern Alps is removed via weathering; then carried across the Canterbury Plains by various braided rivers to the east coast beaches . The second source of sediment is the high cliffs which are located in the hinterland of lagoons . These can be eroded during the occurrence of high river flow or sea storm events . Beaches further south provide nourishment to the northern coast via longshore transport . </P> <P> Hapua have a number of characteristics which includes shifts between a variety of morphodynamic states due to changes in the balance between marine and fluvial processes as well as the antecedent barrier conditions . The MSG barrier constantly changes size and shape as a result of the longshore drift . Water stored in the hapua drains to the coast predominately though an outlet; although it can also seep through the barrier depending on the permeability of the material . </P> <P> Changes in the level of the lagoon water do not occur as a result of saltwater or tidal intrusion . Water in a hapua is predominately freshwater originating from the associated river . Hapua are non-estuarine, there is no tidal inflow however the tide does have an effect on the level of water in the lagoon . As the tide reaches its peak, the lagoon water has a much smaller amount of barrier to permeate through so the lagoon level rises . This is related to a physics theory known as hydraulic head . The lagoon level has a similar sinusoidal wave shape as the tide but reaches its peak slightly later . In general, any saltwater intrusion into the hapua will only occur during a storm via wave overtopping or sea spray . </P>

What is the difference between a bay and a lagoon