The man became suspicious when his dog remained in the bushes for a conspicuously long time around 2 pm on Tuesday afternoon. He then found the tiny newborn wrapped in a plastic bag, a police spokeswoman said.
There are no clues as to who the mother was, but police said they believe the baby was not there for long before it was found. Whoever gave birth to the child probably did so somewhere else and then abandoned the baby near the river, they added.
Authorities don't yet know whether the baby was alive when it was born. Results of an autopsy are expected on Wednesday.
Meanwhile the case of four dead babies found last week in Berlin's Charlottenburg district will likely never be solved because the corpses are too decayed for accurate forensic testing and the alleged mother committed suicide over the summer, daily Berliner Zeitung reported on Monday.
In recent years there have been a number of high-profile cases of infanticide in Germany, with the most notorious involving a woman jailed for 15 years in 2006 for the manslaughter of eight babies.
Sabine Hilschenz, a divorced, unemployed and alcoholic dental assistant from a depressed area of the ex-communist east of the country, hid the corpses in buckets, flowerpots and an old fish tank at her parents' home.