Estimation of sediment volume in the reservoirs is an important management criterion in water use. Many methods are used for this purpose, including hydrography, remote sensing, hydrometry and mathematical and computer models. The high cost of field methods such as hydrography required other methods to be investigated more seriously. In the present research, the hydrometry method was used to estimate the sediment volume in Karaj Dam Reservoir, located on the southern slope of Mount Alborz of Iran. The estimation is based on evaluation of both suspended and bed-load sediments. Although the sediment rating curve method is not common in general, using corrected models based on effective factors of sediment transfer, such as time of measurement, have increased the model efficiency. For this purpose, the daily and annual suspended loads were estimated in two hydrometric stations of Seera and Beylaghan (inlet and outlet hydrometric stations of Karaj Dam) using daily water flow rates and monthly sediment rating equations. Because the empirical methods of bed load sediment did not give acceptable results, the Karaushev curve (which has suitable compatibility with Iranian rivers) was used and the ratio of bed load to suspended load was obtained based on the river slope at hydrometric stations. By using total sediment load and average sediment density, the volumes of sediment were calculated for dam inlet and outlet hydrometric stations. Subtraction of the two volumes gave the stored annual sediment in reservoir of about 406,000 m3. The sediment volume resulting from the hydrography method (from dam primary and secondary area-volume curves) was 416,000 m3, which gave 97% collation, and the trapping efficiency of the Karaj Dam was calculated to be 80%.

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