Unlike in the field of ecology, defining geomorphic rarity of river systems has received little attention. The fluvial geomorphology literature contains some references to some form of rarity, denoting uncommonness (continental to basin scale rarity), unique geo-ecological relationships or vulnerability to alteration or destruction since European occupation of Australia. The majority of definitions of geomorphic rarity rely upon expert opinion (eg. Australian State based initiatives under the HEVAE framework, or use of a sole arbitrary limit of uncommonness, such as a 1% presence figure).
In this paper, we present a process to identify rarity in identified river forms defined using the River Styles Framework. A 15 year program of reach-scale mapping and categorization of 215,000 km of stream length across New South Wales has been completed to about 5th Strahler order, and is being extended to include all 3rd order and higher channels. Completion of NSW River Styles mapping now allows more detailed examination of changes between assessments; current NSW government timeframes include re-assessment to allow evaluation of probable and likely channel adjustments leading to a change in geomorphic condition.
Definition of geomorphic rarity for use in HEVAE has been made on the basis of: uncommonness meaning absolute rarity or endemism, fragmentation where isolated remnants of a particular River Style occur below 2.5% of a named river length, susceptibility where River Styles have very low proportion of reaches in good condition or in moderate condition with rapid potential for geomorphic recovery, and vulnerability to alteration whereby geomorphic forms are under threat – directly or indirectly. In this paper we present the results of this work and discuss how these datasets are being integrated into a single State-wide database, and how this database is being interrogated to identify rare geomorphic forms as a basis for conservation and rehabilitation in NSW.