Re-examination of the erosion and deposition events that culminated in the establishment of the Glenmark soil conservation scheme, Waipara, North Canterbury, New Zealand

Abstract

Pinelea farm is situated on the hill margin and fan piedmont on the north western side of the Omihi valley, Waipara, North Canterbury. The study covers 2000 hectares. The soils occur on several young alluvial fans which bury loess and older alluvium deposits of the adjacent hills margin. Susceptible slopes on the catchment along these hills provide an abundant and easily eroded source of loess, weathered Kowai conglomerates and Mt Brown sandstones.

Soil stratigraphy was used to establish the chronological sequence of the geomorphic surfaces. Within the identified geomorphic surfaces, the distribution of aggrading, composite and compound and simple soil profile forms were related to the patterns and history of episodic sedimentation on fans.

Identification of geomorphic surfaces, including buried geomorphic surfaces, indicates that the most recent episode of erosion and deposition in the 1940's and 50's was proceeded by earlier events. Judging from the rudimentary soil features of surface and buried soils, and associated multisequal soil profiles, these erosion events may have occurred within the past few 100 years.

Four major periods of alluvial fan aggradation were recognised on Pinelea farm. The oldest surface (GS 1) was an exhumed geomorphic surface on Kowai gravels, GS 2 was a depositional geomorphic surface on Late Pleistocene loess, GS 3 and GS 4 were developed on mid to the late Holocene Weka fan alluvium and on the late Holocene piedmont alluvial fan complex respectively.

Annual flooding at Pinelea farm was due to heavy rain associated with easterly or southerly storms. The flooding often produced severe damage to poplar trees, formed deep holes along the main channels collapsed channel walls on the narrow stream, transported organic debris to the flooding areas on Pinelea and adjacent farms and transported sediments throughout the main stream channel to the flooding areas.

Establishment of soil conservation practises as part of the Glenmark Catchment Control Scheme were effective in reducing soil erosion and controlling flooding events. The maintenance and reworking of soil conservation structures, and the planting of trees must continue to prevent further land degradation of this farm soils.