It is estimated that the total annual discharge of New Zealand rivers is approximately 300 km3, and that another 50 km3 of water is 'stored' in perennial snow and glacier ice (see Chapter 9). This running water is of tremendous importance in New Zealand, both directly and as a result of its energy. For example, almost three-quarters of the country is composed of sedimentary rocks, created by the interaction of the basement geology with tectonic processes and erosion. Fluvial erosion processes have sculpted much of this terrain to produce dramatic landscapes, while deposition of fluvially eroded sediments has formed extensive flood plains that make up about 15% of the landmass. These are used extensively for agriculture and horticulture, the produce from which makes up a significant proportion of the country's exports.