Hawke's Bay's estuaries are classified into two hydrosystem types. Shallow Intertidal Dominated Estuaries (SIDE) include Ahuriri Estuary, Maungawhio Lagoon, and Pōrangahau Estuary. SIDE estuaries may be more sensitive to the addition of nutrients and contaminants than river mouth estuaries as they tend to retain water for longer. Shallow, Short Residence Tidal River Estuaries (SSRTRE, or river mouths) include Wairoa, Waitangi, and Tukituki Estuaries. Water passes quickly through these estuaries to the ocean.