Complex sentence: Gold was discovered in the Yentna District of the upper Susitna Valley in 1898, followed soon thereafter by Gold_placer_claim staking and placer mining operations were reported in the Cache Creek drainage of the Dutch Hills by 1906.
Split into simpler sentences: Gold was discovered in the Yentna District of the upper Susitna Valley in 1898, soon followed soon by claim staking. Placer mining was reported in the Cache Creek drainage of the Dutch Hills by 1906.