'Inconsolable' father who drowned his baby son after jumping into a river while high on cactus powder and cannabis is jailed for seven-and-a-half years

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More than three years after he plunged into a Queensland river cradling his baby son, David Paul Fisher is still haunted by his child's drowning death.

'I cry for my baby boy every night,' he told Brisbane's Supreme Court.

The 42-year-old had taken teething baby Elijah out on the evening of June 23, 2012, hoping a walk would soothe him.

But when Mr Fisher returned home, he was dripping wet, distressed and alone.

The father had fallen into the Logan River, 55km north of the Gold Coast, while cradling his son in his arms, and let go.

The six-month-old's body was found downstream the next morning.

Mr Fisher on Monday pleaded guilty to manslaughter, as well as two drug-related offences.

Paramedics who attended that night found him 'inconsolable,' Crown Prosecutor Todd Fuller said.

Elijah (pictured) drowned after his father David Fisher fell into a Queensland river in 2012 while trying to console him. Mr Fisher pleaded guilty to manslaughter and two drug-related offences on Monday

He said Mr Fisher had spent the morning taking drugs including synthetic cannabis, which he later told police he used because it 'took away the pain.'

These substances - including cactus powder - exacerbated underlying mental conditions and caused severe, psychotic delusions, the court heard.

But defence barrister John Allen said Mr Fisher showed considerable remorse and has even earned the forgiveness of his now ex-wife, Lauren.

The court heard the pair had adopted an alternative lifestyle and spent time travelling throughout Australia and New Zealand in a converted bus prior to Elijah's death.

'It's clear that the emotional consequences to a large number of people have been enormous. I acknowledge they have also been enormous for you,' Justice Ann Lyons said.

She cited psychological reports which stated Mr Fisher was at times 'overwhelmed by grief' at having caused his son's death in the throes of a hallucinatory episode and was otherwise a loving parent.

Nevertheless, Justice Lyons noted the crown's submission young Elijah was extremely vulnerable when Fisher released him into the swirling river.

The Logan River (pictured), 55km north of the Gold Coast, Queensland, where Elijah drowned.The six-month-old's body was found downstream the next morning

She sentenced Mr Fisher to seven-and-a-half years' imprisonment, to be eligible for parole after three-and-a-half years.

Considering more than 1,200 days already spent in custody, he could be released before the end of the year.

Since losing their child and Mr Fisher's arrest, Lauren and their four daughters have continued their nomadic travels around Australia in their rainbow coloured bus.

Lauren documents the death of Elijah, her continued travels and learning how to be a single parent after 16 years together in her blog Sparkling Adventures.

In a post titled 'I have a son,' Lauren celebrates what would have been his second birthday.

'Before he was seven months old, Elijah drowned. He was loved and then he was gone. Dead,' she writes. 

'So each night I snuggle in closer to my four precious daughters, thankful that they are each still by my side. For I have experienced the fragility of life and know that no outcome is guaranteed.'