More than 1,000 of 4,000 families have already been evicted or have moved from Boeung Kak lake area.
Richardson's visit follows a high-profile trip by US Secretary of State Hillary Clinton in November.
Members of the Thailand Patriot Network gathered in front of the Cambodian Embassy to call for the release of the group.
Meanwhile, the National Election Committee has been accused of weak enforcement of the Election law.
The reporters killed include those who supported the opposition parties, as well as the pro-government newspaper Koh Santepheap.
The charges carry a sentence of three to six months in jail and a fine of up to $500.
An independent analyst says Cambodia could see growth as high as 10 percent in coming years if governance is improved.
Every year, thousands of Cambodians migrate to work abroad in places like Thailand or Malaysia.
In a statement to the Assembly, Prime Minister Hun Sen said Cambodia had a 30-fold difference in trade imbalance.
The $495 million dam, which China Huandian Corporation will control, is expect to produce 338 megawatts by 2015.