1. Geomorphological Evolution & The Ancestral Waugal

The Derbal Yerrigan (the Swan River estuary) constitutes the primary arterial drainage system of south-western Western Australia, discharging into the Indian Ocean at Fremantle (Walyalup).^[1] Under Bibbullmun customary jurisprudence, the geomorphological contours of the river—its meandering curves, deep ancestral pools (Boyembillup), and rocky scarps—were carved during deep time by the Waugal, the great ancestral water serpent whose living spiritual presence animates the underground aquifers and surface flows.^[1][2]

Geologically, the Derbal Yerrigan represents a drowned river valley (ria) formed during the post-glacial marine transgression between 10,000 and 6,500 years before present (BP).^[2][3] Prior to the melting of continental ice sheets, the river flowed across an exposed coastal plain as a fast-moving freshwater torrent into a marine shoreline situated up to twenty kilometres west of modern Rottnest Island (Wadjemup).^[3][4] As sea levels rose, marine waters inundated the lower river channel, transforming the gorge into an expansive, tide-dominated estuarine basin characterised by brackish lagoons and rich intertidal mudflats.^[4][5]

2. Fluvial Stratigraphy & Pleistocene Chronosequence at Upper Swan (38,000+ BP)

The chronological antiquity of human custodial stewardship along the Derbal Yerrigan is established by stratified Pleistocene archaeological deposits at Upper Swan.^[6] Radiocarbon and thermoluminescence dating of excavated alluvial clay terraces conducted by R. H. Pearce and M. Barbetti confirmed continuous human occupation spanning more than 38,000 years BP.^[6][7]

The stone artifact assemblages excavated from deep clay layers include scrapers, notched flakes, and quartz reduction cores sealed beneath successive flood deposits.^[7][8] These findings prove that Bibbullmun ancestors managed the riparian ecosystems of the river through dramatic paleoclimatic oscillations, including the hyper-arid Last Glacial Maximum (21,000–18,000 BP), maintaining stable hydrological adaptations while European populations were confined to periglacial refugia.^[8][9]

3. Estuarine Aquaculture & The Matagarup Fish Weirs

The estuarine hydrology of the Derbal Yerrigan supported an intensely productive seasonal aquaculture economy.^[10] At Matagarup (Heirisson Island), where the river widened into braided mudflats and shallow causeways connecting northern Mooro lands with southern Beeliar territory, Whadjuk custodians constructed monumental brushwood and stone fish traps (marragup).^[10][11]

During late summer (Bunuru) and early autumn (Djeran), large migrations of sea mullet (Mugil cephalus), cobbler (Cnidoglanis macrocephalus), and Perth herring (Nematalosa vlaminghi) moved up the estuary to feed in shallow sand shoals.^[11][12] Whadjuk engineers coordinated cooperative harvesting drives: men drove fish schools into narrow stone channels while pod-calling whistles enlisted the assistance of estuarine dolphins (Tursiops truncatus) to trap the catch against the shoals.^[12][13] Strict customary quotas prohibited the capture of breeding stock, ensuring sustainable ecological recruitment year after year.^[13][14]

4. Colonial Encroachment, Navigation Seizures & Unceded River Title

The entry of Captain James Stirling's exploration vessel in 1827 and the establishment of the Swan River Colony in 1829 initiated the immediate, violent alienation of the Derbal Yerrigan's foreshores.^[15] Colonial authorities surveyed riverfront ribbons into narrow rectangular Crown grants, denying Whadjuk custodians physical access to their ancestral fishing weirs, freshwater springs, and sacred Waugal ceremonial sites.^[15][16]

Resistance leaders such as Yagan and Midgegooroo asserted unceded proprietary ownership over the waterway, declaring that the taking of colonial flour and livestock constituted lawful restitution for the invaders' destruction of riverine food systems.^[16][17] Despite martial dispossession and subsequent twentieth-century dredging and industrial infill, the Federal Court confirmed in Bennell v State of Western Australia [2006] FCA 1243 that traditional customary connection to the river continues unbroken.^[17][18] Under international law and the doctrine established in Mabo [No 2], Bibbullmun sovereign title over the Derbal Yerrigan remains permanent, inalienable, and unceded.^[18][19]