Aug 15 (Reuters) -Basis bids for soybeans shipped by barge to the U.S. Gulf Coast were flat to weaker on Thursday as forecasts for ample crop supplies weighed down the market, traders said.

* Soybean CIF values were weak despite better-than-expected weekly export sales and firm barge freight rates.

* Barge freight costs for nearby and mostly deferred shipments have climbed as lower water levels on the Mississippi River sparked concerns about a third straight year of harvest-season transportation disruptions, traders said. BG/US

* The Mississippi River gauge at Memphis is showing water levels at the critical choke point 16 feet below a month ago, with forecasts for a further decline in the coming days. Barge lines have already implemented draft restrictions on vessels.

* Chinese importers continue to inquire about U.S. new-crop soybean shipments. Some Octoberand November shipments have been booked from Gulf Coast terminals this week. Other buyers are waiting on further price declines after the U.S. Department of Agriculture (USDA) projected a record U.S. soy crop this week.

* The USDA said net U.S. new-crop soybean export sales last week totaled 1.344 million metric tons, above trade estimates for up to 1 million tons. EXP/SOY

* CIF Gulf soybean barges loaded in August were bid 2 cents lower at 66 cents over CBOT November SX24 soybean futures. September soybean barge bids were also down 2 cents at 57 cents over November futures.

* FOB soy export premiums for early September loading at the Gulf were unchanged at about 108 cents over November futures. Last-half September shipments were offered steady at around 90 cents over futures.

* For corn, CIF Gulf barges loaded in August were bid 2 cents lower at 55 cents over CBOT September CU24 futures.

* FOB Gulf September export premiums were unchanged at 82 cents over September corn futures.

Reporting by Karl Plume in Chicago; Editing by Shilpi Majumdar

For displays of CIF basis, click on the codes in brackets: U.S. CIF Gulf soybeans GRYM U.S. CIF Gulf corn GRYN U.S. CIF Gulf SRW wheat GRYO U.S. CIF Gulf HRW wheat GRYP
For displays of FOB basis, please click on the following codes in brackets: U.S. FOB Gulf corn GRZC U.S. FOB Gulf soybeans GRZD U.S. FOB Gulf SRW wheat GRZE U.S. FOB Gulf HRW wheat GRZF LINKS Corn prices page: cpurl://apps.cp./cms/?pageid=corn-cash-market
Wheat prices page: cpurl://apps.cp./cms/?pageid=wheat-cash-market
U.S. barge freight BG/US
U.S. export sales estimates USDA/EST
U.S. grain export summary GRA/U
Brazil soybean export prices SYBS
Brazil corn export prices SYBX
Argentina grain prices GRA/ARG
Russia grain prices GRA/RU